New helper_array_transpose api function

Required for issue #15774
Fix #15774: unable to simultaneously upload > 5 files
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
Tests: added function for globals declaration

MantisBT configs (defined in config_defaults_inc.php) are not
defined in the global scope when running PHPUnit. Because of this,
including 'core.php' does not work.

All 'global' configs must therefore be specifically declared as such
when API functions are needed to run tests.