ci: exclude spec files from copilot review (#4919)

Author: tadjik1

.github/instructions/spec-tests.instructions.md

@@ -0,0 +1,8 @@
+---
+applyTo:
+  - "test/spec/**/*.json"
+  - "test/spec/**/*.yml"
+  - "test/spec/**/README.md"
+---
+These are unified spec test files copied from the mongodb/specifications repo.
+Do not suggest changes to these files. They must match the upstream spec exactly.