runtime: regression test for issue 50936

Add a regression test for issue 50936 which coerces the runtime into
frequent execution of the cgocall dropg/execute curg assignment race by
making many concurrent cgo calls eligible for P retake by sysmon. This
results in no P during exitsyscall, at which point they will update curg
and will crash if SIGPROF arrives in between updating mp.curg and
mp.curg.m.

This test is conceptually similar to the basic cgo callback test in
aprof.go but with additional concurrency and a sleep in C.

On my machine this test fails ~5% of the time prior to CL 382079.

For #50936.

Change-Id: I21b6c7f2594f9a615a64580ef70a88b692505678
Reviewed-on: https://go-review.googlesource.com/c/go/+/382244


Trust: Michael Pratt <mpratt@google.com>
Run-TryBot: Michael Pratt <mpratt@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>
Reviewed-by: Cherry Mui <cherryyz@google.com>