selftests: tc-testing: remove spurious './' from Makefile

Patchwork CI didn't like the extra './', so remove it.

Signed-off-by: Pedro Tammela <pctammela@mojatatu.com>
Acked-by: Jamal Hadi Salim <jhs@mojatatu.com>
Link: https://lore.kernel.org/r/20231129222424.910148-3-pctammela@mojatatu.com


Signed-off-by: Jakub Kicinski <kuba@kernel.org>