PSR2/UseDeclaration: move parse error test case to its own file

Update comment to indicate that this is an intentional parse error. This was
implied by the original comment instructing that it must be the last line
in the test case file.

Author: biinari

src/Standards/PSR2/Tests/Namespaces/UseDeclarationUnitTest.1.inc

@@ -38,6 +38,3 @@ enum SomeEnum
 $x = $foo ? function ($foo) use /* comment */ ($bar): int {
     return 1;
 } : $bar;
-
-// Testcase must be on last line in the file.
-use

src/Standards/PSR2/Tests/Namespaces/UseDeclarationUnitTest.18.inc

@@ -0,0 +1,4 @@
+<?php
+
+// Intentional parse error. Live coding resilience.
+use