Add doctest

The doctest assures that there is no output when calling
`getfixture("crate")`.

Author: chaudum

tests/layer.rst

@@ -0,0 +1,13 @@
+============
+Pytest Crate
+============
+
+Pytest fixtures can also be used in doctests, however, doctests always capture
+the outout of any method calls.
+
+Therefore the pytest fixture must not print anything to stdout/stderr but
+rather use a logger.
+
+.. code-block::
+
+   >>> crate = getfixture("crate")