Fix wxFileName unit test under Unix after recent changes

Don't use GetVolumeSeparator() to combine UNC or GUID volumes with
paths, this doesn't work because this kind of paths doesn't contain
colons at all.

Update the documentation to mention this.

interface/wx/filename.h

@@ -877,6 +877,14 @@ public:
 
     /**
         Returns the string separating the volume from the path for this format.
+
+        Note that for @c wxPATH_DOS paths this string can only be used for
+        single-character volumes representing the drive letters, but not with
+        the UNC or GUID volumes (see their description in GetVolume()
+        documentation). For this reason, its use should be avoided, prefer
+        using wxFileName constructor and Assign() overload taking the volume
+        and the path as separate arguments to combining the volume and the path
+        into a single string using the volume separator between them.
     */
     static wxString GetVolumeSeparator(wxPathFormat format = wxPATH_NATIVE);