]erlang] link test resources instead of copying them

Summary:
There are 2 reasons for this:
1. it's more efficient to link then to copy these files.
2. some tests want to update test fixtures when given a specific run flag, and must know the original file path.

Reviewed By: alanz

Differential Revision: D67855901

fbshipit-source-id: b681c627524553bc7392f23b370841c474968f4c

prelude/erlang/erlang_utils.bzl

@@ -145,7 +145,7 @@ def _file_mapping_impl(ctx: AnalysisContext) -> list[Provider]:
         for file in files:
             target_path = paths.normalize(target_path)
             out_path = paths.normalize(paths.join(target_path, file.basename))
-            out = ctx.actions.copy_file(
+            out = ctx.actions.symlink_file(
                 out_path,
                 file,
             )