Improving name generation in MSTest

[aaronpowell]

Summary of the changes (Less than 80 chars)

• Null checking when generating snapshot name from params (MS Test data driven tests fail if data param contains null #178)
• Using the DisplayName of a TestDataMethod if provided in MSTest
• Removing illegal characters from the generated file name

[RohrerF]

/azp run

[azure-pipelines]

Azure Pipelines successfully started running 1 pipeline(s).