Parse test: Fortify tests.

* Differentiate expressions used as AST nodes to avoid undetected
  errors in argument order of AST constructors.
* Make tests more readable, by using semantically correct code.
AST test: Use skip decorator instead of SkipTest Exception.
AST test: Make assertion more sure-y.
AST test: Beutify and fix some pylint warnings.
Tests: Import sure at package level, as is recommended.
Type test: Make testing less verbose.
Type test: Fix some pylint warnings.