runtime/pprof: TestMutexProfile flaky on OpenBSD

[bradfitz]

Flake https://storage.googleapis.com/go-build-log/c7f32b9b/openbsd-amd64-60_09545cfb.log

--- FAIL: TestMutexProfile (0.02s)
	pprof_test.go:532: expected 6 lines, got 3 "--- mutex:\ncycles/second=2487572969\nsampling period=1"
		--- mutex:
		cycles/second=2487572969
		sampling period=1
FAIL
FAIL	runtime/pprof	1.295s

[ianlancetaylor]

Dup of #19139?

[bradfitz]

Yup, thanks.