**Added:**

* xonsh_builtins, xonsh_execer fixtures in conftest.py

**Changed:**

* many test cases to use fixtures and parametrization

**Deprecated:**

* yield statements (nose style) and for loops in tests

**Removed:**

* for loops and yield statements in test cases, unused imports

**Fixed:**

* redundant and 'leaky' tests in nose

**Security:** None