Added a few tests for variable names

This includes tests for Unicode variables names, braced variables (not
yet passing, hence Explicit) as well as the other special cases
mentioned in §2.3.2: $?, $_, $^.
Expanded Unicode variable tests

Also collapsed them into a single [Test] with several [TestCase]s
Merge pull request #120 from ygra/variable-expression-tests

Tests for variable names