selftests/bpf: Add timer stress test in NMI context

Add stress tests for BPF timers that run in NMI context using perf_event
programs attached to PERF_COUNT_HW_CPU_CYCLES.

The tests cover three scenarios:
- nmi_race: Tests concurrent timer start and async cancel operations
- nmi_update: Tests updating a map element (effectively deleting and
  inserting new for array map) from within a timer callback
- nmi_cancel: Tests timer self-cancellation attempt.

A common test_common() helper is used to share timer setup logic across
all test modes.

The tests spawn multiple threads in a child process to generate
perf events, which trigger the BPF programs in NMI context. Hit counters
verify that the NMI code paths were actually exercised.

Signed-off-by: Mykyta Yatsenko <yatsenko@meta.com>
Signed-off-by: Alexei Starovoitov <ast@kernel.org>
Signed-off-by: Andrii Nakryiko <andrii@kernel.org>
Link: https://lore.kernel.org/bpf/20260201025403.66625-8-alexei.starovoitov@gmail.com

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