[test] Allow skipTestIfFn to apply to entire classes for skipIfNoSBHeaders

Some test cases are already marked @skipIfNoSBHeaders, but they make use of SBAPI headers in test setup. The setup will fail if the headers are missing, so it is too late to wait until the test case to apply the skip annotation.

In addition to allowing this to apply to entire classes, I also changed all the existing annotations from test cases to test classes where necessary/appropriate.

Reviewed By: DavidSpickett

Differential Revision: https://reviews.llvm.org/D138181