.gitattributes: Minimize golden test JSON files in GitHub PR

This causes golden JSON files used for tests to be hidden by default
in the GitHub PR review page. They are hidden in the same way that
"large diffs" are hidden by default.

The reviewer can then concentrate on the substance of the PR, and then
inspect JSON files by clicking on the "Load diff" link.

.gitattributes

@@ -11,3 +11,9 @@ nix/sources.nix linguist-generated=true
 
 # Hide the Cabal freeze file
 cabal.project.freeze linguist-generated=true
+
+# Hide golden JSON files used for tests
+# - they clog up the GitHub PR review page.
+# Also set the "binary" merge mode for Git,
+# because textual merging of auto-generated JSON rarely works well.
+lib/core/test/data/Cardano/Wallet/Api/*.json linguist-generated=true merge=binary