testsuite: Increase delay in conc059

As noted in #19179 (closed), conc059 can sometimes fail due to too short of a delay in the its Haskell threads. Address this by increasing the delay by an order of magnitude to 5 seconds.

While I'm in town I refactored the test to eliminate a great deal of unnecessary platform dependence, eliminate use of the deprecated usleep, and properly handle interruption by signals.

Fixes #19179 (closed).