fix(builtin): declare size=small on generated_file_test

The test itself just runs diff, so it's always fast. This prevents warnings under bazel test --test_verbose_timeout_warnings
Note that the **kwargs are passed to both the binary and test targets here, so users can't pass this attribute

internal/generated_file_test/generated_file_test.bzl

@@ -36,6 +36,7 @@ def generated_file_test(name, generated, src, substring_search = False, src_dbg
             loc % generated,
         ],
         data = data,
+        size = "small",
         **kwargs
     )