selftests/bpf: skip nmi test when perf hw events are disabled

Some setups (e.g. virtual machines) might run with hardware perf events
disabled. If this is the case, skip the test_send_signal_nmi test.

Add a separate test involving a software perf event. This allows testing
the perf event path regardless of hardware perf event support.

Signed-off-by: Ilya Leoshkevich <iii@linux.ibm.com>
Acked-by: Andrii Nakryiko <andriin@fb.com>
Signed-off-by: Alexei Starovoitov <ast@kernel.org>