selftests/timens: Add a simple perf test for clock_gettime()

Output on success:
1..4
 ok 1 host:	clock:  monotonic	cycles:	 148323947
 ok 2 host:	clock:   boottime	cycles:	 148577503
 ok 3 ns:	clock:  monotonic	cycles:	 137659217
 ok 4 ns:	clock:   boottime	cycles:	 137959154
 # Pass 4 Fail 0 Xfail 0 Xpass 0 Skip 0 Error 0

Output with lack of permissions:
 1..4
 ok 1 host:	clock:  monotonic	cycles:	 145671139
 ok 2 host:	clock:   boottime	cycles:	 146958357
 not ok 3 # SKIP need to run as root

Output without support of time namespaces:
 1..4
 ok 1 host:	clock:  monotonic	cycles:	 145671139
 ok 2 host:	clock:   boottime	cycles:	 146958357
 not ok 3 # SKIP Time namespaces are not supported

Co-developed-by: Dmitry Safonov <dima@arista.com>
Signed-off-by: Andrei Vagin <avagin@gmail.com>
Signed-off-by: Dmitry Safonov <dima@arista.com>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Link: https://lore.kernel.org/r/20191112012724.250792-34-dima@arista.com