[ots] Make OTSContext destructor vertual

Since the class has virtual functions, its destructor ought to be
virtual.

Issue #41.
[build] Restore building cff_type2_charstring_test

That was lost when the scons build was dropped in d321ea2.

Does not build yet, though, because of the internal API change in
da8d278.

Related to #40.
[test] Fix building cff_type2_charstring_test

Now we get one test failure, investigating.

Related to #40.
[test] Drop outdated check

We support dotsection operator since 8d97ca2.

Related to #40.