selftests: tc-testing: timeout on unbounded loops

In the spirit of failing early, timeout on unbounded loops that take
longer than 20 ticks to complete. Such loops are to ensure that objects
created are already visible so tests can proceed without any issues.

If a test setup takes more than 20 ticks to see an object, there's
definetely something wrong.

Signed-off-by: Pedro Tammela <pctammela@mojatatu.com>
Reviewed-by: Simon Horman <horms@kernel.org>
Acked-by: Jamal Hadi Salim <jhs@mojatatu.com>
Link: https://lore.kernel.org/r/20231117171208.2066136-6-pctammela@mojatatu.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>