Disable parallel for smoke test

[zpoint]

We find some buildkite host OOM recently, could be our tests create high memory usage

Before this PR

We can see 16 workers [1 item] in the log

sky) zpoint:~/codes/skypilot (dev/zeping/disable_parallel_for_smoke_test)$ pytest tests/smoke_tests/test_examples.py::test_nemorl --aws -k accelerator0 --remote-server -v
D 10-30 15:32:59 skypilot_config.py:247] using default user config file: ~/.sky/config.yaml
D 10-30 15:32:59 skypilot_config.py:514] Config loaded from /home/zpoint/.sky/config.yaml:
D 10-30 15:32:59 skypilot_config.py:514] {}
D 10-30 15:32:59 skypilot_config.py:514] 
D 10-30 15:32:59 skypilot_config.py:521] Config syntax check passed for path: /home/zpoint/.sky/config.yaml
D 10-30 15:32:59 skypilot_config.py:274] using default project config file: .sky.yaml
D 10-30 15:32:59 skypilot_config.py:666] client config (before task and CLI overrides): 
D 10-30 15:32:59 skypilot_config.py:666] {}
D 10-30 15:32:59 skypilot_config.py:666] 
=============================================================================== test session starts ================================================================================
platform linux -- Python 3.10.18, pytest-8.4.1, pluggy-1.6.0
rootdir: /home/zpoint/codes/skypilot
configfile: pyproject.toml
plugins: env-1.1.5, anyio-4.9.0, asyncio-1.0.0, xdist-3.7.0
asyncio: mode=strict, asyncio_default_fixture_loop_scope=function, asyncio_default_test_loop_scope=function
16 workers [1 item]       
[nemorl] Test started. Log: less -r /tmp/nemorl-tmpc9lbq.log
[nemorl] Overriding API server endpoint: http://host.docker.internal:41540
[nemorl] Failed (returned 1).

After this PR

No more 16 workers [1 item] in the log

(sky) zpoint:~/codes/skypilot (dev/zeping/disable_parallel_for_smoke_test)$ pytest tests/smoke_tests/test_examples.py::test_nemorl --aws -k accelerator0 --remote-server -v
D 10-30 15:33:24 skypilot_config.py:247] using default user config file: ~/.sky/config.yaml
D 10-30 15:33:24 skypilot_config.py:514] Config loaded from /home/zpoint/.sky/config.yaml:
D 10-30 15:33:24 skypilot_config.py:514] {}
D 10-30 15:33:24 skypilot_config.py:514] 
D 10-30 15:33:24 skypilot_config.py:521] Config syntax check passed for path: /home/zpoint/.sky/config.yaml
D 10-30 15:33:24 skypilot_config.py:274] using default project config file: .sky.yaml
D 10-30 15:33:24 skypilot_config.py:666] client config (before task and CLI overrides): 
D 10-30 15:33:24 skypilot_config.py:666] {}
D 10-30 15:33:24 skypilot_config.py:666] 
['tests/smoke_tests/test_examples.py::test_nemorl']
=============================================================================== test session starts ================================================================================
platform linux -- Python 3.10.18, pytest-8.4.1, pluggy-1.6.0
rootdir: /home/zpoint/codes/skypilot
configfile: pyproject.toml
plugins: env-1.1.5, anyio-4.9.0, asyncio-1.0.0, xdist-3.7.0
asyncio: mode=strict, asyncio_default_fixture_loop_scope=function, asyncio_default_test_loop_scope=function
collected 1 item                                                                                                                                                                   

tests/smoke_tests/test_examples.py [nemorl] Test started. Log: less -r /tmp/nemorl-zd7dk38q.log

Tested (run the relevant ones):

• Code formatting: install pre-commit (auto-check on commit) or bash format.sh
• Any manual or new tests for this PR (please specify below)
• All smoke tests: /smoke-test (CI) or pytest tests/test_smoke.py (local)
• Relevant individual tests: /smoke-test -k test_name (CI) or pytest tests/test_smoke.py::test_name (local)
• Backward compatibility: /quicktest-core (CI) or pytest tests/smoke_tests/test_backward_compat.py (local)

[zpoint]

/smoke-test --remote-server

[aylei]

LGTM, thanks @zpoint !

[zpoint]

/smoke-test --remote-server

[zpoint]

/smoke-test --remote-server

[aylei]

Looks like the percentage > 20% now after we limit the cpu cores of remote server to 4 and decrease the base usage:

KEY                BASELINE     ACTUAL   INCREASE       %
--
  | 721:worker:short   161.8 MB   161.9 MB    +0.1 MB   +0.1%
  | 833:server         165.8 MB   187.3 MB   +21.5 MB  +13.0%
  | 746:server         165.6 MB   171.1 MB    +5.5 MB   +3.3%
  | 745:server         165.5 MB   171.2 MB    +5.6 MB   +3.4%
  | 683:server         166.9 MB   171.5 MB    +4.5 MB   +2.7%
  | 727:worker:short   162.2 MB   162.2 MB    +0.0 MB   +0.0%
  | 725:worker:short   222.6 MB   223.0 MB    +0.4 MB   +0.2%
  | 1056:worker:long     0.0 MB   244.7 MB  +244.7 MB   +inf%
  | TOTAL             1210.5 MB  1492.9 MB  +282.4 MB  +23.3%
  | FINFO:conftest:Last worker finished, cleaning up container...
  | sky-remote-test-buildkite-generic-remote-server3
  | sky-remote-test-buildkite-generic-remote-server3

Should be fine to tune the percentage threshold cc @kevinmingtarja

[zpoint]

/smoke-test --remote-server -k test_big_file_upload_memory_usage