t/op/threads.t: unit test 10 frequently fails on threaded builds on NetBSD

During smoke-testing, t/op/threads.t often (but not always) fails in threaded builds on NetBSD and other OSes. See, for example, these smoke-test reports:

http://perl5.test-smoke.org/report/99678
http://perl5.test-smoke.org/report/108475

The smoke-test reports, which are from NetBSD VMs set up by Carlos and myself, typically only say:

Test failures:
~~ ../t/op/threads.t ...... FAILED Bad plan. You planned 30 tests but ran 9.
[stdio] -Duseithreads

'FAILED Bad plan' usually indicates that the program has crashed. The 'ran 9' suggests that the crash occurred during unit test 10. Here is t/op/threads.t test 10:

# [perl #45053] Memory corruption with heavy module loading in threads
#
# run-time usage of newCONSTSUB (as done by the IO boot code) wasn't
# thread-safe - got occasional coredumps or malloc corruption
watchdog(180, "process");
{
    local $SIG{__WARN__} = sub {};   # Ignore any thread creation failure warnings
    my @t;
    for (1..10) {
        my $thr = threads->create( sub { require IO });
        last if !defined($thr);      # Probably ran out of memory
        push(@t, $thr);
    }
    $_->join for @t;
    ok(1, '[perl #45053]');
}

... where watchdog is a function defined in t/test.pl for the purpose of "timeout the entire test file".

This test was added in October 2007 in response to RT 45053, which is now https://github.com/Perl/perl5/issues/9008, "Memory corruption with heavy module loading in threads."

Typically, when I run this test file on a threaded build on NetBSD outside the context of smoke-testing, it PASSes.

Is there any way this test could be made less fragile? @jdhedden

Thank you very much.
Jim Keenan

Start with test 10 in t/op/threads.t and the watchdog implementation in t/test.pl. Reproduce the failure on a threaded NetBSD build and compare the linked smoke-test reports with standalone runs. Done means the threaded test no longer intermittently aborts with a bad plan while preserving its coverage and expected test count.