selftests/bpf: Report the real error from libarena parallel workers

Several CI runs failed in the libarena parallel tests with -4 (-EINTR) [1],
which says nothing about what actually went wrong.

Two workers can fail like this:

  worker 1: gives up, e.g. the rendezvous times out, sets test_abort and
            returns its own error (-ETIMEDOUT)
  worker 2: sees test_abort and returns -EINTR

-EINTR only means "someone else already gave up", so it carries no
information. Which of the two gets reported depends on the order
pthread_join() collects them, because

	err = err ?: (long)thread_ret;

keeps the first non-zero value and drops the rest. When the -EINTR worker
comes first, the error describing the actual failure is lost.

Skip -EINTR entirely: a worker only returns it once another worker has
already reported the real error, so report and log only the real errors.

It is still unclear whether the timeouts come from CI load or from a
problem in the test itself. Report the error accurately first, so the next
failure can be diagnosed.

[1]:
https://github.com/kernel-patches/bpf/actions/runs/29867905253/job/88764463566
https://github.com/kernel-patches/bpf/actions/runs/29878191901/job/88794845824

Signed-off-by: Jiayuan Chen <jiayuan.chen@linux.dev>
Reviewed-by: Emil Tsalapatis <emil@etsalapatis.com>
Link: https://lore.kernel.org/bpf/20260723061347.398591-1-jiayuan.chen@linux.dev
Signed-off-by: Kumar Kartikeya Dwivedi <memxor@gmail.com>

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