"error_log" ini should allow VFS paths

[mvorisek]

Description

related with #17817

repro: https://3v4l.org/AEfvC

Currently, it is not possible to set error_log ini config to a non-FS path.

When error log needs to be asserted in an app, real tmp file needs to be created. This is slow and RW FS is required.

I would be happy if VFS/custom stream wrapper could be supported in error_log.

[mvorisek]

Might I know how feasible this is?

It would not only make testing substantially faster, but it would also allow to detect the output origin backtrace.