Add a way for a method to decide to skip and not run (not a failure, not a success)

[clach04]

One idea in the method at a decision point simply issue:

return SKIP;

For other implementations see:

• http://junit.sourceforge.net/javadoc/org/junit/Ignore.html
• https://docs.python.org/3/library/unittest.html#skipping-tests-and-expected-failures

Where the "skipped" and "ignored" are the names used.