Merge branch 'selftests/bpf: fix flaky send_signal test'

Yonghong Song says:

====================

The bpf selftest send_signal() is flaky for its subtests trying to
send signals in softirq/nmi context. To reduce flakiness, the
signal-targetted process priority is boosted, which should minimize
preemption of that process and improve the possibility that
the underlying task in softirq/nmi context is the bpf_send_signal()
wanted task.

Patch #1 did a refactoring to use ASSERT_* instead of old CHECK macros.
Patch #2 did actual change of boosting priority.

Changelog:
  v1 -> v2:
    remove skip logic where the underlying task in interrupt context
    is not the intended one.
====================

Signed-off-by: Andrii Nakryiko <andrii@kernel.org>