Incorrect test result for permission check of uploads directory if it doesn't exist yet

[ocean90]

wp_get_upload_dir() returns the theoretical path of the uploads directory. When there was no upload yet the test returns an incorrect result (not writeable) because the directory doesn't exist.

Switching to wp_upload_dir(), which creates the directory on the fly, ensures a proper test result.

[afragen]

This looks like an easy win.

[ocean90]

In 46228:

Site Health: Replace wp_get_upload_dir() with wp_upload_dir().

wp_get_upload_dir() returns the theoretical path of the uploads directory. When there was no upload yet the test returns an incorrect result (not writeable) because the directory doesn't exist. Switching to wp_upload_dir(), which creates the directory on the fly, ensures a proper test result.

Fixes #47958.