test/run: limit parallelism to 1 for cross-exec builds

This matters for NaCl, which seems to swamp my 4-core MacBook Pro otherwise.
It's not a correctness problem, just a usability problem.

LGTM=bradfitz
R=bradfitz
CC=golang-codereviews
https://golang.org/cl/98600046

test/run.go

@@ -71,15 +71,15 @@ const maxTests = 5000
 func main() {
 	flag.Parse()
 
-	// Disable parallelism if printing
-	if *verbose {
-		*numParallel = 1
-	}
-
 	goos = os.Getenv("GOOS")
 	goarch = os.Getenv("GOARCH")
 	findExecCmd()
 
+	// Disable parallelism if printing or if using a simulator.
+	if *verbose || len(findExecCmd()) > 0 {
+		*numParallel = 1
+	}
+
 	ratec = make(chan bool, *numParallel)
 	rungatec = make(chan bool, *runoutputLimit)
 	var err error