internal/copyright: close files

Fixes golang#70507

Change-Id: I2ab1f221ebd45fedc70d1fded29b084147eadd28
Reviewed-on: https://go-review.googlesource.com/c/go/+/630815
Auto-Submit: Ian Lance Taylor <[email protected]>
Reviewed-by: Dmitri Shuralyov <[email protected]>
Reviewed-by: Tim King <[email protected]>
LUCI-TryBot-Result: Go LUCI <[email protected]>
Reviewed-by: Dmitri Shuralyov <[email protected]>

src/internal/copyright/copyright_test.go

@@ -49,6 +49,7 @@ func TestCopyright(t *testing.T) {
 			t.Error(err)
 			return nil
 		}
+		defer f.Close()
 		n, err := f.Read(buf)
 		if err != nil && err != io.EOF {
 			t.Error(err)