cmd/go: add test case for cgo coverage

This is a test case for CL 34680044.

Fixes #6333.

LGTM=bradfitz
R=golang-codereviews, bradfitz, minux.ma
CC=golang-codereviews
https://golang.org/cl/71230049

src/cmd/go/test.bash

@@ -568,6 +568,16 @@ TEST coverage runs
 ./testgo test -short -coverpkg=strings strings regexp || ok=false
 ./testgo test -short -cover strings math regexp || ok=false
 
+TEST coverage with cgo
+d=$(TMPDIR=/var/tmp mktemp -d -t testgoXXX)
+./testgo test -short -cover ./testdata/cgocover >$d/cgo.out 2>&1 || ok=false
+cat $d/cgo.out
+if grep 'coverage: 0.0%' $d/cgo.out >/dev/null; then 
+	ok=false
+	echo no coverage for cgo package
+	ok=false
+fi
+
 TEST cgo depends on syscall
 rm -rf $GOROOT/pkg/*_race
 d=$(TMPDIR=/var/tmp mktemp -d -t testgoXXX)

src/cmd/go/testdata/cgocover/p.go

+package p
+
+/*
+void
+f(void)
+{
+}
+*/
+import "C"
+
+var b bool
+
+func F() {
+	if b {
+		for {
+		}
+	}
+	C.f()
+}

src/cmd/go/testdata/cgocover/p_test.go

+package p
+
+import "testing"
+
+func TestF(t *testing.T) {
+	F()
+}