cmd/go: make go test -i -o x.test actually write x.test

Fixes #17078.

Change-Id: I1dfb71f64361b575ec461ed44b0779f2d5cf45fc
Reviewed-on: https://go-review.googlesource.com/31352Reviewed-by: Quentin Smith <quentin@golang.org>

src/cmd/go/go_test.go

@@ -1703,6 +1703,16 @@ func TestGoTestDashOWritesBinary(t *testing.T) {
 	tg.wantExecutable(tg.path("myerrors.test"+exeSuffix), "go test -o myerrors.test did not create myerrors.test")
 }
 
+func TestGoTestDashIDashOWritesBinary(t *testing.T) {
+	tg := testgo(t)
+	defer tg.cleanup()
+	tg.parallel()
+	tg.makeTempdir()
+	tg.run("test", "-v", "-i", "-o", tg.path("myerrors.test"+exeSuffix), "errors")
+	tg.grepBothNot("PASS|FAIL", "test should not have run")
+	tg.wantExecutable(tg.path("myerrors.test"+exeSuffix), "go test -o myerrors.test did not create myerrors.test")
+}
+
 // Issue 4568.
 func TestSymlinksList(t *testing.T) {
 	switch runtime.GOOS {

src/cmd/go/test.go

@@ -435,6 +435,11 @@ func runTest(cmd *Command, args []string) {
 	testStreamOutput = len(pkgArgs) == 0 || testBench ||
 		(testShowPass && (len(pkgs) == 1 || buildP == 1))
 
+	// For 'go test -i -o x.test', we want to build x.test. Imply -c to make the logic easier.
+	if buildI && testO != "" {
+		testC = true
+	}
+
 	var b builder
 	b.init()