docs: include test dependencies when developing pyOpenSSL (#1271)

If someone is running `pip install -e` to develop on pyOpenSSL,
presumably they want to be able to run the tests too.

Author: davidben

INSTALL.rst

@@ -7,7 +7,7 @@ To install pyOpenSSL::
 
 If you are installing in order to *develop* on pyOpenSSL, move to the root directory of a pyOpenSSL checkout, and run::
 
-  $ pip install -e .
+  $ pip install -e .[test]
 
 
 .. warning::