WebAssert::optionExists and WebAssert::optionNotExists now find only exact matches

Formerly WebAssert::optionExists() and WebAssert::optionNotExists() asserted partial matches for the $option parameter. This behavior was not advertised in the docblocks and it led to random test fails in core.

Now these methods only assert exact matches for the $option parameter. The $select parameter still allows partial matches.

If a test was previously accepting:
$assert->optionExists('menu[menu_parent]', 'Main navigation');
add some extra carrots <,>on to the assert like:
$assert->optionExists('menu[menu_parent]', '<Main navigation>');