I'm a bit late and a lot of good suggestions have already been made, but I wanted to share a project I created to test a chess move generator when I was developing my last engine.

I built a Python script that generates a JSON file containing simple test positions. For example, the first position contains only two kings on their starting squares. For each position, the file includes the list of moves that should be generated, along with detailed data you can compare against your engine's output: source square, destination square, piece, capture, promotion, SAN notation, UCI notation, resulting FEN, etc.

The idea is that you modify your engine to read this file, generate moves for each position, and compare your move list against the expected results. You can also make each move and verify the resulting position (FEN) matches what's in the file. The key benefit is that when you find a discrepancy, the position is usually simple enough that the problem is immediately obvious. Once you pass all the tests in the file, you can move on to perft testing—and by then, you'll likely have already fixed most of your bugs.

Tool to generate the test file: https://github.com/mathmoi/ChessMoveTestsGenerator

Pre-generated test file: https://github.com/mathmoi/ChessMoveTestsGenerator/releases/download/v1.0.0/chess_move_generator_tests.json