proc: unlock OS thread on exit from handlePtraceFuncs (#2972)

On FreeBSD and OpenBSD, the use of runtime.LockOSThread is resulting in segfaults
within the Go runtime (see https://github.com/golang/go/issues/52394) - while it
should not be necessary, calling runtime.UnlockOSThread upon exit from
handlePtraceFuncs avoids this issue and allows the tests to run correctly.

pkg/proc/native/proc.go

@@ -271,6 +271,13 @@ func (dbp *nativeProcess) handlePtraceFuncs() {
 	// all commands after PTRACE_ATTACH to come from the same thread.
 	runtime.LockOSThread()
 
+	// Leaving the OS thread locked currently leads to segfaults in the
+	// Go runtime while running on FreeBSD and OpenBSD:
+	//   https://github.com/golang/go/issues/52394
+	if runtime.GOOS == "freebsd" || runtime.GOOS == "openbsd" {
+		defer runtime.UnlockOSThread()
+	}
+
 	for fn := range dbp.ptraceChan {
 		fn()
 		dbp.ptraceDoneChan <- nil