selftests/bpf: Add BPF-helper test for CLOCK_TAI access

Add BPF-helper test case for CLOCK_TAI access. The added test verifies that:

 * Timestamps are generated
 * Timestamps are moving forward
 * Timestamps are reasonable

Signed-off-by: Kurt Kanzenbach <kurt@linutronix.de>
Link: https://lore.kernel.org/r/20220809060803.5773-3-kurt@linutronix.de


Signed-off-by: Alexei Starovoitov <ast@kernel.org>