nmtst/valgrind: allow calling 'run-test-valgrind.sh' script directly

nmtst/valgrind: allow calling 'run-test-valgrind.sh' script directly

When you want to run valgrind for a test, you either had to
invoke valgrind manually, or doing it via `make check` (provided
you configured --with-valgrind).

Make it more convenient to run valgrind by passing the test
to run to the "run-test-valgrind.sh" wrapper.

This also allows to pass -p/-s to the test, which is not possible
during `make check` because selecting tests conflicts with "--tap".
The following invocations are largely equivalent and work as
expected:

  $ ./tools/run-test-valgrind.sh ./src/platform/tests/test-link-linux -p /link/software/detect/vlan

  $ NMTST_DEBUG=no-debug,p=/link/software/detect/vlan ./tools/run-test-valgrind.sh ./src/platform/tests/test-link-linux