Add --fork-list arg to testgen

[hwwhww]

This PR adds a --fork-list arg to the test generator to only generate test vectors of the specific forks.

For instance, to only generator deneb tests, we can modify run_generator command in Makefile:

python3 main.py -o $(CURRENT_DIR)/$(TEST_VECTOR_DIR)

to

python3 main.py -o $(CURRENT_DIR)/$(TEST_VECTOR_DIR) --fork-list deneb