tc-testing: Allow test cases to be skipped

By adding a check for an optional key/value pair to the test case
data, individual test cases may be skipped to prevent tdc from
aborting a test run due to setup or teardown failure.

If a test case is skipped, it will still appear in the results
output to allow for a consistent number of executed tests in each
run. However, the test will be marked as skipped.

This support for skipping extends to any plugins that may generate
additional results for each executed test.

Signed-off-by: Lucas Bates <lucasb@mojatatu.com>
Signed-off-by: David S. Miller <davem@davemloft.net>