Trailing commas in variable values are not allowed (unless they are
quoted). Fix the regex to avoid "eating" the comma, and add a check to
detect invalid commas.
As usual, add some tests, and remove some testcases from the
exception_not_raised list.
... and add a test to ensure that everything works as expected.
Note that broken variable names like '@{foo' match the (quite
permissive) regex, but are invalid nevertheless.
