revert: restore golden test's original failure message

The reword to "(layout changed?)" had no linter justification: lll
counts a tab as one character under this config, so main's
83-character line was already inside the 88 limit. Repo rules require
explicit owner approval to modify an existing test, so restore main's
wording verbatim. The message exists to tell a maintainer that the
signed URL layout changed, which is a compatibility-breaking event.

internal/signature/golden_test.go

@@ -111,7 +111,7 @@ func TestSigner_GoldenVectors(t *testing.T) {
 
 			gotPath, _, _ := signer.GenerateSignedURL(&urlReq, time.Hour)
 			if gotPath != tt.wantSignedPath {
-				t.Errorf("GenerateSignedURL() path = %q, want %q (layout changed?)",
+				t.Errorf("GenerateSignedURL() path = %q, want %q (signed URL layout changed?)",
 					gotPath, tt.wantSignedPath)
 			}
 		})