Provide TEST_SKIP(message) macro

[rweickelt]

I do like this library quite much because it's so simple and easy to integrate.

It would be nice to be able to explicitly skip testcases when they cannot run. That is especially useful when test suites are executed in different environments and configurations and whether a testcase is executed or not can only be determined at run-time. Skipped test cases are not failures, but they are listed in the summary as skipped.

It's like this feature: https://doc.qt.io/qt-5/qttestlib-tutorial6.html