runtime: TestThreadExhaustion core dump with maymorestack=mayMoreStackMove

[prattmic]

#!watchflakes
post <- pkg == "runtime" && test == "TestThreadExhaustion" && `signal`

On https://go.dev/cl/456096:

https://storage.googleapis.com/go-build-log/a1f2cf23/linux-amd64-longtest_82c0e1c3.log

##### maymorestack=mayMoreStackMove
--- FAIL: TestThreadExhaustion (0.01s)
    crash_test.go:58: /workdir/tmp/go-build866713881/testprog.exe ThreadExhaustion: signal: trace/breakpoint trap (core dumped)
    crash_test.go:246: output does not start with "runtime: program exceeds 10-thread limit\nfatal error: thread exhaustion":
FAIL
FAIL	runtime	360.108s
FAIL

cc @golang/runtime @aclements