Improve CoreCLR test runner outside Windows

Improve CoreCLR test runner outside Windows

CoreCLR tests will need to be built on Windows and copied over to a non-Windows machine. Then, coreclr/tests/runtest.sh along
with some switches can be used to run the tests.

Changes:
- Run tests in parallel by default (use --sequential to force sequential mode)
- Update Windows test build to produce the Windows core overlay that includes test dependencies that are not built
- Add a number of switches (see coreclr/tests/runtest.sh for details) to provide locations of:
  - The Windows test build
  - The CoreCLR native test build
  - Either:
    - This:
      - The core layout (including all of the below)
    - Or:
      - The CoreCLR build
      - mscorlib.dll
      - The CoreFX build
      - The CoreFX native build
- Using information from above, build or use the core layout, and copy native test binaries to the respective test directories
- Add --testDir to specify an inclusive list of test directories to run
- Add text files to exclude tests:
  - coreclr/tests/testsUnsupportedOutsideWindows.txt - Tests that are not supported outside Windows
  - coreclr/tests/testsFailingOutsideWindows.txt - Tests that are known to fail outside Windows and are temporarily disabled while they are investigated
- Add --runFailingTestsOnly to run only the tests listed in coreclr/tests/testsFailingOutsideWindows.txt to verify known failures
- Produce xUnit-style output for the CI, output goes into <Windows test build folder>/coreclrtests.xml