Tests: fix whitespace, comments and phpDoc
Tests: fix inclusion of TestConfig.php

This way tests can be called individually
Tests: String - use string API functions

Remove hardcoded copy of string_sanitize_url()

Added require_once calls for necessary API and includes. This needed a
change in TestConfig.php to add the Mantis Root directory to the include
path.
Tests: Strings - fix broken test cases (&amp; => &)

Commit c59ad8a60f316d8cda10b578280c86668838e9e9 changed the use of
ampersands in internally generated URLs
Tests: add new Helper API test script

Currently only covers helper_array_transpose() function