cmd/cover: add go.mod file in lineDupDir

This fixes TestFuncWithDuplicateLines (introduced in CL 167257)
in module mode.

Updates #30746
Updates #30228

Change-Id: I7b3e7192ae23f855c373e881389874ff6ffd49ad
Reviewed-on: https://go-review.googlesource.com/c/go/+/167740
Run-TryBot: Bryan C. Mills <bcmills@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Andrew Bonventre <andybons@golang.org>

src/cmd/cover/cover_test.go

@@ -540,6 +540,9 @@ func TestFuncWithDuplicateLines(t *testing.T) {
 		t.Fatal(err)
 	}
 
+	if err := ioutil.WriteFile(filepath.Join(lineDupDir, "go.mod"), []byte("module linedup\n"), 0444); err != nil {
+		t.Fatal(err)
+	}
 	if err := ioutil.WriteFile(lineDupGo, []byte(lineDupContents), 0444); err != nil {
 		t.Fatal(err)
 	}
@@ -554,7 +557,7 @@ func TestFuncWithDuplicateLines(t *testing.T) {
 
 	// testcover -func=TMPDIR/linedup.out
 	cmd = exec.Command(testcover, "-func", lineDupProfile)
-	cmd.Dir = testTempDir
+	cmd.Dir = lineDupDir
 	run(cmd, t)
 }