bug(fill): detect and raise an error if `state_test()` or `blockchain_test()` is executed more than once in one test function execution

For example, see https://github.com/jochem-brouwer/execution-spec-tests/blob/dccaec30b31625c9eb4e6395d6caa854307204cd/tests/byzantium/precompiles/test_05_modexp.py which calls `state_test` in a for loop in a test function.

While every `state_test()` execution, respectively, test vector in `data`, will get filled, only the last generated test fixture will ever get stored and written to disk by the fixture collector.