selftests: net: vrf_strict_mode_test: add support to select a test to run

Add a boilerplate test loop to run all tests in
vrf_strict_mode_test.sh. Add a -t flag that allows a selected test to
run. Remove the vrf_strict_mode_tests function which is now unused.

Signed-off-by: Jaehee Park <jhpark1013@gmail.com>
Reviewed-by: David Ahern <dsahern@kernel.org>
Link: https://lore.kernel.org/r/20220429164658.GA656707@jaehee-ThinkPad-X1-Extreme


Signed-off-by: Paolo Abeni <pabeni@redhat.com>