tests: add unit tests for header and template funcs

Much of this is being tested with the existing file-based tests in
testdata/*, but this moves us toward a much more targetted and simpler
test structure.

main.go

@@ -334,5 +334,5 @@ func hasLicense(b []byte) bool {
 	}
 	return bytes.Contains(bytes.ToLower(b[:n]), []byte("copyright")) ||
 		bytes.Contains(bytes.ToLower(b[:n]), []byte("mozilla public")) ||
-		bytes.Contains(bytes.ToLower(b[:n]), []byte("SPDX-License-Identifier"))
+		bytes.Contains(bytes.ToLower(b[:n]), []byte("spdx-license-identifier"))
 }