tools: tc-testing: Refactor test-runner

Split the test_runner function into the loop part (test_runner)
and the contents (run_one_test) for maintainability.
It makes it a little easier to catch exceptions
in an individual test, and keep going (and flush a bunch
of tap results for the skipped tests).

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