This resolves problems when installing Moodle
and addons when preventexecpath active.
Please note the admin_setting_configfile is still
incorrectly restricted - this will be resolved by
MDL-40949.
This commit gets rid of the old lib/tests/other/executable.php
file completely because it was only executable under unix OSs
and moves the check to be performed against some mimetex binaries
that are known to be executable and bundled with every installation.
Note this was discovered because the weekly rolling process changed
the execution permissions of the "fixture" file and, at the end
we decided to get rid of it completely, folowing the plan above.
