selftests/bpf: Add case to test bpf_in_interrupt()

Add a timer test case to test 'bpf_in_interrupt()'.

cd tools/testing/selftests/bpf
./test_progs -t timer_interrupt
462     timer_interrupt:OK
Summary: 1/0 PASSED, 0 SKIPPED, 0 FAILED

Signed-off-by: Leon Hwang <leon.hwang@linux.dev>
Link: https://lore.kernel.org/r/20250903140438.59517-3-leon.hwang@linux.dev


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