selftests/net: plug rxtimestamp test into kselftest framework

Run rxtimestamp as part of TEST_PROGS. Analogous to other tests, add
new rxtimestamp.sh wrapper script, so that the test runs isolated
from background traffic in a private network namespace.

Also ignore failures of test case #6 by default. This case verifies
that a receive timestamp is not reported if timestamp reporting is
enabled for a socket, but generation is disabled. Receive timestamp
generation has to be enabled globally, as no associated socket is
known yet. A background process that enables rx timestamp generation
therefore causes a false positive. Ntpd is one example that does.

Add a "--strict" option to cause failure in the event that any test
case fails, including test #6. This is useful for environments that
are known to not have such background processes.

Tested:
make -C tools/testing/selftests TARGETS="net" run_tests

Signed-off-by: Tanner Love <tannerlove@google.com>
Acked-by: Willem de Bruijn <willemb@google.com>
Signed-off-by: David S. Miller <davem@davemloft.net>