Remove obsolete doc/en/test directory

Apparently this was an old version of the docs, which has long been superseded
pytest-dev · nicoddemus · Jun 13, 2018 · Jun 11, 2018 · Jun 12, 2018 · Jun 12, 2018
commit d6e7b1a21bb2f9cf9019cbfee1f3bfad160891dd
diff --git a/doc/en/test/attic.rst b/doc/en/test/attic.rst
diff --git a/doc/en/test/config.html b/doc/en/test/config.html
diff --git a/doc/en/test/dist.html b/doc/en/test/dist.html
diff --git a/doc/en/test/extend.html b/doc/en/test/extend.html
diff --git a/doc/en/test/index.rst b/doc/en/test/index.rst
diff --git a/doc/en/test/mission.rst b/doc/en/test/mission.rst