Document Translate Toolkit use of pytest and plugins

This is based on docs originally part of Pootle PR#57 (https://github.com/translate/pootle/pull/57) - incorporating review comments there, the testing docs are now in Translate Toolkit; PR#57 has been modified to just reference these.

Note that although "explicit is better than implicit" in Python code, I'm not convinced this is true in reST formatted documentation, which should be as readable as possible in source form as well as processed output; accordingly I have not added .. code-block:: python (as it is the default) or .. code-block:: bash (as there is no significant highlighting on a simple command line example) - the net effect of adding those would not change the output at all, but would reduce readability of the source.