Allow sub-directories for Fixture folder

Allow sub-directories for Test/Fixture folder (for app. and plugin.)
The directory separator is '/'.
A file ``MyClassFixture.php` in the folder `Fixture/MyFolder` will be included like this :
`'app.MyFolder/my_class'`
Merge pull request #2 from Tounu/patch-3

Allow sub-directories for Fixture folder
Creation of a method to parse fixture path

Creation of `_parseFixturePath`
Merge pull request #3 from Tounu/patch-4

Creation of a method to parse fixture path
Php cs fixed
Merge pull request #4 from Tounu/patch-5

Php cs fixed
Merge pull request #3005 from Tounu/2.5

Allow sub-directories in Fixture folder