selftests/timens: added selftest for /proc/stat btime

Test that btime value of /proc/stat is as expected in the time namespace
using a simple parser to get btime from /proc/stat.

Signed-off-by: Michael Weiß <michael.weiss@aisec.fraunhofer.de>
Reviewed-by: Andrei Vagin <avagin@gmail.com>
Acked-by: Thomas Gleixner <tglx@linutronix.de>
Acked-by: Christian Brauner <christian.brauner@ubuntu.com>
Link: https://lore.kernel.org/r/20201027204258.7869-4-michael.weiss@aisec.fraunhofer.de