Pass paths to test script from build-and-test

[lbussell]

This fixes a bug which prevented you from running the correct tests when passing paths to build-and-test.ps1

Other changes:

• Avoid running the pre-build tests twice by removing it from the test categories if it is ran before the build step.
• Don't run the build twice if we aren't building samples.
• Adjust the default test categories to make building and testing faster by default.

[lbussell]

Added an extra feature that I had been trying for a while - you can run very specific tests by passing -CustomTestFilter "VerifyFxDependentAppScenario". Or more specific, "AspnetImageTests.VerifyFxDependentAppScenario". It is based on the fully qualified name of the test method.