selftests/timens: Add procfs selftest

Check that /proc/uptime is correct inside a new time namespace.

Output on success:
 1..1
 ok 1 Passed for /proc/uptime
 # Pass 1 Fail 0 Xfail 0 Xpass 0 Skip 0 Error 0

Output with lack of permissions:
 1..1
 not ok 1 # SKIP need to run as root

Output without support of time namespaces:
 1..1
 not ok 1 # SKIP Time namespaces are not supported

Co-developed-by: Andrei Vagin <avagin@gmail.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/20191112012724.250792-32-dima@arista.com