selftests/bpf: Add test for unpinning htab with internal timer struct

selftests/bpf: Add test for unpinning htab with internal timer struct

Add test to verify that unpinning hash tables containing internal timer
structures does not trigger context warnings.

Each subtest (timer_prealloc and timer_no_prealloc) can trigger the
context warning when unpinning, but the warning cannot be triggered
twice within a short time interval (a HZ), which is expected behavior.

Signed-off-by: KaFai Wan <kafai.wan@linux.dev>
Acked-by: Yonghong Song <yonghong.song@linux.dev>
Link: https://lore.kernel.org/r/20251008102628.808045-3-kafai.wan@linux.dev
Signed-off-by: Alexei Starovoitov <ast@kernel.org>