Test support for multi instances of XWiki is randomly broken

Regression caused by https://github.com/xwiki/xwiki-platform/commit/add97355dee0db332168f6bdaf80df0a79651655#diff-b0e982be257cca40e15a1beed3d038df035f7034037cd7485e74f188aa03ba6bR161

In case of multi instances test you have folders looking that like "target/xwiki", "target/xwiki-1", etc. and findXWikiDirectoryInTarget can end up selecting on any of them as base folder to which it then add again "-1", "-2", etc.