Tests: Add filtering tests for uncovered cases

While investigating the code coverage of the filtering feature, a couple
of possible tests cases were uncovered:

Error tests:
* Strings can't be IR root node
* Unary ! not allowed on string type
* Comparison with string type not allowed
* Logical operator not allowed with string types
* Nesting of binary operator not allowed

Valid tests:
* Cover all left/right operands permutations with
  fields ref. and numeric values.

Signed-off-by: Christian Babeux <christian.babeux@efficios.com>
Signed-off-by: David Goulet <dgoulet@efficios.com>