cmd/go/internal/script: check lack of error for non-waiting cmds

In the script engine, if a command does not return a Wait function and
it succeeds, we won't call checkStatus. That means that commands that
don't have a wait function, have a "!" indicating that they are
supposed to fail, and then succeed will spuriously not fail the script
engine test even they were supposed to fail but didn't.

Change-Id: Ic88c3cdd628064d48f14a8a4a2e97cded48890fb
Reviewed-on: https://go-review.googlesource.com/c/go/+/451284
Reviewed-by: Michael Matloob <matloob@golang.org>
Reviewed-by: Bryan Mills <bcmills@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>
Run-TryBot: Michael Matloob <matloob@golang.org>

src/cmd/go/internal/script/engine.go

@@ -564,11 +564,14 @@ func (e *Engine) runCommand(s *State, cmd *command, impl Cmd) error {
 	}
 
 	wait, runErr := impl.Run(s, cmd.args...)
-	if runErr != nil {
+	if wait == nil {
+		if async && runErr == nil {
+			return cmdError(cmd, errors.New("internal error: async command returned a nil WaitFunc"))
+		}
 		return checkStatus(cmd, runErr)
 	}
-	if async && wait == nil {
-		return cmdError(cmd, errors.New("internal error: async command returned a nil WaitFunc"))
+	if runErr != nil {
+		return cmdError(cmd, errors.New("internal error: command returned both an error and a WaitFunc"))
 	}
 
 	if cmd.background {

src/cmd/go/testdata/script/list_issue_56509.txt

@@ -4,9 +4,10 @@
 # leading the package matching code to think there were Go files in the
 # directory.)
 
+cd bar
 go list ./...
 ! stdout .
-
+cd ..
 
 [short] skip