cmd/go: convert TestFmtLoadErrors to a script test

This test failed in a pending CL, and I would rather debug it as a script.

Change-Id: I0ae7486a9949bea40d5dd36afe6919f86f14dfa7
Reviewed-on: https://go-review.googlesource.com/c/go/+/206499
Run-TryBot: Bryan C. Mills <bcmills@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>

src/cmd/go/go_test.go

@@ -5704,14 +5704,6 @@ func TestInstallDeps(t *testing.T) {
 	tg.mustExist(p1)
 }
 
-func TestFmtLoadErrors(t *testing.T) {
-	tg := testgo(t)
-	defer tg.cleanup()
-	tg.setenv("GOPATH", filepath.Join(tg.pwd(), "testdata"))
-	tg.runFail("fmt", "does-not-exist")
-	tg.run("fmt", "-n", "exclude")
-}
-
 func TestGoTestMinusN(t *testing.T) {
 	// Intent here is to verify that 'go test -n' works without crashing.
 	// This reuses flag_test.go, but really any test would do.

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

@@ -0,0 +1,19 @@
+env GO111MODULE=off
+
+! go fmt does-not-exist
+
+go fmt -n exclude
+stdout 'exclude[/\\]x\.go'
+stdout 'exclude[/\\]x_linux\.go'
+
+-- exclude/empty/x.txt --
+-- exclude/ignore/_x.go --
+package x
+-- exclude/x.go --
+// +build linux,!linux
+
+package x
+-- exclude/x_linux.go --
+// +build windows
+
+package x