CRM_Utils_File::isChildPath - Fix checking of non-existent paths

CRM_Utils_File::isChildPath - Fix checking of non-existent paths

Suppose you use call `isChildPath()` with these options:

* `$parent='/nonexistent'`
* `$child='/home/me/exists.txt'`
* `$checkRealPath=TRUE`

Before
=====

Reports that `/home/me/exists.txt` is a child of `/nonexistent`

After
=====

Reports that `/home/me/exists.txt` is NOT a child of `/nonexistent`

Comments
========

I added six test cases for how `isChildPath()` behaves when using
`$checkRealPath=TRUE`.  The first five cases behave the same with or without
the patch.  The patch fixes the last case.