Add some checks for merged test groups (#89521)

Add some checks for merged test groups (#89521)

- CLRTest.Execute.Batch.targets
  - Fix CLRTestExecutionArguments (allows passing a filter argument to <mergedgroup>.cmd
  - Fix indenting in generated script
- XUnitWrapperGenerator
  - Move some generated code into (shared) ITestInfo.cs
  - Also display exception on stdout on failure
  - Add check that projects in merged test groups don't contain entry points
    - Can be a sign of code that is expected to run but won't be
    - Or can be a [Fact]/etc-less entry point in a ReqProcIso project but would then need to be fixed if ReqProcIso were removed
- Directory.Build.targets
  - Run XUnitWrapperGenerator on test projects in merged groups
  - Can't simply replace the conditional because we have XUnit-style tests outside of merged groups
- Update tests to conform to above checks plus a bit of opportunistic cleanup