test-pthread-rwlock failure on Pop!_OS 22.04 LTS

[Paul Eggert]

When running './gnulib-tool --test nstrftime" on my laptop's Pop!_OS 
22.04 LTS, test-pthread-rwlock went into what appeared to be a CPU bound 
infinite loop. It would have drained the battery if I hadn't been 
plugged into the wall. The process had many threads active. I eventually 
gave up and killed the test.

Pop!_OS is downstream of Ubuntu, and this version is using glibc 
2.35-0ubuntu3.8. uname -srvmpi reports "Linux 6.9.3-76060903-generic 
#202405300957~1718348209~22.04~7817b67 SMP PREEMPT_DYNAMIC Mon J x86_64 
x86_64 x86_64". The laptop uses an Intel Core i5-1335U.

I suggest redoing the pthread tests so that they run in far less than 
the current 10 minutes of real time before abandoning the test. 1 or 2 
seconds would be nice. If that's not realistic, perhaps we should add a 
new option --expensive to ./gnulib-tool, to cause it to also run 
expensive tests, and mark the pthread tests as expensive.