go/expect: remove testdata go.mod to go.fake.mod

go.mod defines a module boundary, making the go/expect test fail when
run as a dependency of another module because the testdata is missing.

Rename go.mod to go.fake.mod to keep it in the parent module. go/expect
will still parse it as a mod file.

Fixes #57783.

Change-Id: Ib17af058e26042bf5148e9aa11fc3d7f9b9c6e7f
Reviewed-on: https://go-review.googlesource.com/c/tools/+/461942
Auto-Submit: Michael Pratt <mpratt@google.com>
Reviewed-by: Bryan Mills <bcmills@google.com>
Run-TryBot: Michael Pratt <mpratt@google.com>
Reviewed-by: Robert Findley <rfindley@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>

go/expect/expect_test.go

@@ -43,7 +43,7 @@ func TestMarker(t *testing.T) {
 			},
 		},
 		{
-			filename:    "testdata/go.mod",
+			filename:    "testdata/go.fake.mod",
 			expectNotes: 2,
 			expectMarkers: map[string]string{
 				"αMarker": "αfake1α",

go/expect/testdata/go.fake.mod

@@ -0,0 +1,9 @@
+// This file is named go.fake.mod so it does not define a real module, which
+// would make the contents of this directory unavailable to the test when run
+// from outside the repository.
+
+module αfake1α //@mark(αMarker, "αfake1α")
+
+go 1.14
+
+require golang.org/modfile v0.0.0 //@mark(βMarker, "require golang.org/modfile v0.0.0")