Ensure M object files are created in bats test/subtest output directory and not in tests/fixtures

• Avoids hard-to-debug test failures due to concurrent .o file format changes
• While at this, also reordered the ydb_routines and related YDB env var initialization to happen at init_test() time (called once and at start of subtest) instead of create_db() time (can be called multiple times per subtest).