cmd/go: make "go test" recognize -exec flag again.

Fixes #12144.

Change-Id: I112c6517371215c9797db8f1dffca0f3047c39e3
Reviewed-on: https://go-review.googlesource.com/13633Reviewed-by: Russ Cox <rsc@golang.org>

src/cmd/go/testflag.go

@@ -36,6 +36,7 @@ var testFlagDefn = []*testFlagSpec{
 	{name: "cover", boolVar: &testCover},
 	{name: "covermode"},
 	{name: "coverpkg"},
+	{name: "exec"},
 
 	// passed to 6.out, adding a "test." prefix to the name if necessary: -v becomes -test.v.
 	{name: "bench", passToTest: true},