docs: update README_TESTS.md to highlight Makefile usage

Update test documentation to recommend make commands as the primary
method for running tests, with manual pytest commands as a fallback.

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude <[email protected]>

Author: jontsai

README_TESTS.md

@@ -1,8 +1,30 @@
 # Running Tests
 
-## Quick Start
+## Quick Start (Using Make)
 
-The easiest way to run the catalog extra fields tests:
+The easiest way to run tests:
+
+```bash
+# First time setup - create virtual environment
+make venv
+source .venv/bin/activate
+
+# Install dependencies
+make install
+
+# Run all tests
+make test
+
+# Run just the catalog vendor tests
+make test-vendor
+
+# Run tests with coverage
+make test-cov
+```
+
+## Manual Test Commands
+
+If you prefer to run pytest directly:
 
 ```bash
 # Activate virtual environment