selftests/bpf: Add absolute timer test

Add test for the absolute BPF timer under the existing timer tests. This
will run the timer two times with 1us expiration time, and then re-arm
the timer at ~35s in the future. At the end, it is verified that the
absolute timer expired exactly two times.

Signed-off-by: Tero Kristo <tero.kristo@linux.intel.com>
Link: https://lore.kernel.org/r/20230302114614.2985072-3-tero.kristo@linux.intel.com


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