Fixup 216a6fb3: fix broken xml syntax

Fixup 216a6fb3: fix broken xml syntax

This commit addresses two issues:

1/ the embarassing one: the member SetMode::old_text_level_ was
   declared as bool instead of int. This means that is was definitely
   not a proper backup variable!

2/ a robustness issue: replace two consecutive test for isTest() by a
   boolean veriable that is used twice. This makes sure that <mrow>
   cannot be output without the corresponding </mrow>.

Part of bug #13069.