tests: Use proper word splitting when executing tests

tests: Use proper word splitting when executing tests

Use the shell special variable "$@" instead of the inferior $* to
execute the test command in ts_valgrind.  The expansion of "$@" respects
proper word splitting and makes it possible to pass the command empty
arguments.  It might also prevent surprises with quoting in corner
cases.

Tested that `make check` passes.

Valgrind run with `make check TS_OPTS='--nonroot --memcheck'` passes.

Signed-off-by: Filipe Brandenburger <filbranden@google.com>