cmd/go: "go test -c -test.bench=XX fmt" shouldn't hang

Fixes #6480.

R=golang-dev, rsc
CC=golang-dev
https://golang.org/cl/13925043

src/cmd/go/test.bash

@@ -408,7 +408,7 @@ if ! grep -q '^hello world' hello.out; then
 	cat hello.out
 	ok=false
 fi
-rm -rf $d
+rm -rf $d hello.out
 
 TEST go test -cpuprofile leaves binary behind
 ./testgo test -cpuprofile strings.prof strings || ok=false
@@ -613,6 +613,13 @@ fi
 rm -rf $d
 unset GOPATH
 
+TEST 'Issue 6480: "go test -c -test.bench=XXX fmt" should not hang'
+if ! ./testgo test -c -test.bench=XXX fmt; then
+	echo build test failed
+	ok=false
+fi
+rm -f fmt.test
+
 # clean up
 if $started; then stop; fi
 rm -rf testdata/bin testdata/bin1

src/cmd/go/test.go

@@ -449,7 +449,7 @@ func runTest(cmd *Command, args []string) {
 	}
 
 	// Force benchmarks to run in serial.
-	if testBench {
+	if !testC && testBench {
 		// The first run must wait for all builds.
 		// Later runs must wait for the previous run's print.
 		for i, run := range runs {