Fix performance of string regression (#6832)

Fix performance of string regression (#6832)

Regression cmu-dis-0707-3.smt2 has been timing out when using an ASan
build after commit a4f38d6. Before that
commit --strings-exp implicitly enabled --fmf-bound. After the
commit, the solver was supposed to apply the same reasoning but only to
interal quantifiers and without enabling --fmf-bound. However, the
commit missed some options checks that now also have to check whether
--strings-exp is enabled. This commit updates those option checks.
With this fix, we get a slightly different value for bug590.smt2 after
replying unknown. This commit updates the regression to be more lenient
with the value returned.