Tweak tests, allowing some minor behavior changes
Two changes in behavior are allowed due to the new system checking the
result of a parse before trying to call eval() rather than just calling
eval on as many toplevel statements as possible:
* Syntax errors no longer get into back traces as frames - the line
information is in the ParseError instead.
* `include_string()` throws any ParseError before try to call eval(), so
LoadError is not thrown when the string is malformed.
