cmd/go: handle -ccflags in 'go test'

CL 89050043 only allows -ccflags for 'go test', this
CL really handles the flag like the other -??flags.
Many thanks to Dobrosław Żybort for pointing this out.
Fixes #7810 (again).

LGTM=iant, matrixik
R=golang-codereviews, iant, matrixik
CC=golang-codereviews
https://golang.org/cl/89230044

src/cmd/go/testflag.go

@@ -160,6 +160,11 @@ func testFlags(args []string) (packageNames, passToTest []string) {
 			if err != nil {
 				fatalf("invalid flag argument for -%s: %v", f.name, err)
 			}
+		case "ccflags":
+			buildCcflags, err = splitQuotedFields(value)
+			if err != nil {
+				fatalf("invalid flag argument for -%s: %v", f.name, err)
+			}
 		case "gcflags":
 			buildGcflags, err = splitQuotedFields(value)
 			if err != nil {