selftests: tc-testing: preload all modules in kselftests

While running tdc tests in parallel it can race over the module loading
done by tc and fail the run with random errors.
So avoid this by preloading all modules before running tdc in kselftests.

Signed-off-by: Pedro Tammela <pctammela@mojatatu.com>
Reviewed-by: Simon Horman <horms@kernel.org>
Signed-off-by: David S. Miller <davem@davemloft.net>