[Ruby 3.4] Add spec for GC.config

Includes generic tests and tests for MRI's default implementation of GC.

If a Ruby implementation does not want to expose implementation or configuration, a minimal method can be this:
```ruby
def GC.config(options = nil)
  return { implementation: "none" } if options.nil

  raise ArgumentError unless Hash === options
  if options.key?(:implementation)
    raise ArgumentError, 'Attempting to set read-only key "Implementation"'
  end

  {}
end
```

Author: trinistr

core/gc/config_spec.rb

@@ -0,0 +1,86 @@
+require_relative '../../spec_helper'
+
+ruby_version_is "3.4" do
+  describe "GC.config" do
+    context "without arguments" do
+      it "returns a hash of current settings" do
+        GC.config.should be_kind_of(Hash)
+      end
+
+      it "includes the name of currently loaded GC implementation as a global key" do
+        GC.config.should include(:implementation)
+        GC.config[:implementation].should be_kind_of(String)
+      end
+    end
+
+    context "with a hash of options" do
+      it "allows to set GC implementation's options" do
+        # Can't set anything if there are no options.
+        skip if GC.config(foo_bar_nonexistent_option_for_real!: nil).empty?
+
+        # Remove global keys which are read-only.
+        config = GC.config(foo_bar_nonexistent_option_for_real!: nil)
+        # Try to find a boolean setting to reliably test changing it.
+        key, value = config.find { |_k, v| v == true }
+        skip unless key
+
+        GC.config(key => false).should == config.merge(key => false)
+        GC.config[key].should == false
+        GC.config(key => true).should == config
+        GC.config[key].should == true
+      ensure
+        GC.config(config)
+      end
+
+      it "does not change settings that aren't present in the hash" do
+        GC.config({}).should == GC.config.except(:implementation)
+      end
+
+      it "ignores unknown keys" do
+        GC.config(foo: "bar").should == GC.config.except(:implementation)
+      end
+
+      it "raises an ArgumentError if options include global keys" do
+        -> { GC.config(implementation: "default") }.should raise_error(ArgumentError, 'Attempting to set read-only key "Implementation"')
+      end
+    end
+
+    context "with a non-hash argument" do
+      it "returns current settings if argument is nil" do
+        GC.config(nil).should == GC.config
+      end
+
+      it "raises ArgumentError for all other arguments" do
+        -> { GC.config([]) }.should raise_error(ArgumentError, "ArgumentError")
+        -> { GC.config("default") }.should raise_error(ArgumentError, "ArgumentError")
+        -> { GC.config(1) }.should raise_error(ArgumentError, "ArgumentError")
+      end
+    end
+
+    guard -> { PlatformGuard.standard? && GC.config[:implementation] == "default" } do
+      context "with default GC implementation on MRI" do
+        before do
+          @default_config = GC.config({})
+        end
+
+        after do
+          GC.config(@default_config)
+        end
+
+        it "includes :rgengc_allow_full_mark option, true by default" do
+          GC.config.should include(:rgengc_allow_full_mark)
+          GC.config[:rgengc_allow_full_mark].should be_true
+        end
+
+        it "allows to set :rgengc_allow_full_mark" do
+          GC.config(rgengc_allow_full_mark: false).should == @default_config.merge(rgengc_allow_full_mark: false)
+          GC.config(rgengc_allow_full_mark: true).should == @default_config
+
+          # This key maps truthy and falsey values to true and false.
+          GC.config(rgengc_allow_full_mark: nil).should == @default_config.merge(rgengc_allow_full_mark: false)
+          GC.config(rgengc_allow_full_mark: 1.23).should == @default_config
+        end
+      end
+    end
+  end
+end