test: add test_ prefix to a function name

The test function `g.jit_off_on_macOS_by_default` in `gh_8252` was
silently ignored by the luatest due to its lack of the required
`test_` prefix. This commit renames the function to
`test_jit_off_on_macOS_by_default`, ensuring that it is recognized
and executed by the luatest.

Closes #10210

NO_DOC=codehealth
NO_CHANGELOG=codehealth

(cherry picked from commit eca4f17b3588d38a4d61a71af8371f5ed15de248)