[stable9] Workaround to check htaccess in case of redirects (#25435)

* Workaround to check htaccess in case of redirects

In some setups, the web server will redirect any call to "data/" to the
main page. This causes the XHR to return the 200 HTTP status code and
the body contains the HTML page of the main page / files app.

This fix improves the htaccess failure detection by adding a known
string inside the test file "htaccesstest.txt". If we are able to find
this string, it means that the web server didn't block access to that
file.

* Fix setup check unit test (#25439)