selftests: txtimestamp: allow individual txtimestamp tests.

The wrapper script txtimestamp.sh executes a pre-defined list of testcases
sequentially without configuration options available.

Add an option (-r/--run) to setup the test namespace and pass remaining
arguments to txtimestamp binary. The script still runs all tests when no
argument is passed.

Signed-off-by: Jian Yang <jianyang@google.com>
Acked-by: Willem de Bruijn <willemb@google.com>
Acked-by: Soheil Hassas Yeganeh <soheil@google.com>
Signed-off-by: David S. Miller <davem@davemloft.net>