Enable pinvoke tests for coreclr

[KevinRansom]

I completely failed to do a decent job of getting this in earlier.

Encore une fois.

This contains a number of changes:

1. Removed redundant ; from ilreflect.fs
2. Amend singletestbuildandrun. Lock test directory creation, to ensure tests don't interfere with each other.
3. Add -TestCambridge switch to build.ps1 to run just the cambridge tests.
4. Enable fsi-load and pinvoke tests for coreclr.
5. Pinvoke test script used some non-existent dlls, well ... they used to exist but now are no more. So eliminate tests reliant on them. Add in test.ok semaphore.