[test infrastructure] TextEditTest fails due to lack of AssertionError if assertions are off

[TysonAndre]

Steps to reproduce: set zend.assertions = -1 in the PHP ini, run TextEditTest in phpunit.

See http://php.net/manual/en/function.assert.php#function.assert.expectations

Solutions:

• Change the code to throw OutOfBoundsException (or have an explicit throw statement for AssertionError) instead of using assert() so that the behavior of TextEdit::applyEdits is always consistent. If TextEdit::applyEdits is a public API (is it? I don't see it used anywhere), this may make sense to do.
• Skip the test based on the value of zend.assertions.

[TysonAndre]

Duplicate of #161 , missed that the first time I looked due to searching for AssertionError