These fixes were built for PR dotnet#74886; however, as that PR is so utterly large an unreviewable, I've pulled out the test infra changes for separate review

Changes
- Increase the number of trampolines in the llvm aot compilation process to 20,000 from 10,000 (This avoids running out of them in some of the hardware intrinsics tests
- Add a concept of striping tests when running under GC stress
  - To use this new feature, specify <NumberOfStripesToUseInStress>N</NumberOfStripesToUseInStress> within the merged test assembly. If this value is set, then the tests within that merged test assembly will be run across N different work items instead of 1 when running under any form of GC stress based scenario. At this moment the largest supported value of N is 99
- Emit the testresults.xml file as a file which is exported from the tests. This is useful for debugging testresult.xml parsing failures
- Fix the testresults summary generator to never emit an empty CDATA string. If one is present the parser may fail the parse.
- In the XUnitWrapperGenerator fix the implementation of the Outerloop and ActiveIssue when used with a conditional member.
- Add PlatformDetection.IsMonoLLVMAOT, PlatformDetection.IsMonoLLVMFULLAOT, and PlatformDetection.IsMonoInterpreter boolean properties to the PlatformDetection type for use with the ActiveIssue attribute

… as some documentation on the command line parameters for merged test runner assemblies