Could not write error to log file

While debugging for this issue https://github.com/Codeception/Codeception/issues/4871, I keep getting message `Could not write error to log file`.
When I traced the cause, it's from here.

https://github.com/Codeception/c3/blob/c7348bbc82da82834fe237c5fb754003ac0fe782/c3.php#L36-L43

Apparently the `c3tmp` directory is always emptied by `__c3_clear()` when the test is executed.

https://github.com/Codeception/c3/blob/c7348bbc82da82834fe237c5fb754003ac0fe782/c3.php#L288-L291

This cause problem because `is_writable` returns true if the file exists and is writable.

http://php.net/manual/en/function.is-writable.php

Start in c3.php around lines 36-43, then read __c3_clear() around lines 288-291 and trace how the c3tmp directory is handled during test execution. Reproduce the logging failure described in the issue and verify that the error log remains writable after cleanup.

Written by the indexing model from the issue text.