assumevalid was merged as part of PR 9484, but was not added to the
test_runner, so is not run even as part of the extended tests.
This commit adds assumevalid to the list of tests in test_runner. It
also clarifies the code in assumevalid considerably.
Currently, functional test cases can either pass or fail. There are
occasions when it is helpful to skip tests, for example if the
system they are running on does not meet the requirements for the test.
The rest of the test suite can run without being marked as a failure.
This commit adds framework for tests to skip if their requirements
aren't met.
