remove src/buildtest/ and related build_helper/ test files

This was used to exercise our old virtualenv-like scheme. Now that we
use virtualenv, they're unnecessary. Plus, removing it lets us stop
polluting end-user installs with the extra package (that might
conceivably conflict with some other project that names itself
"buildtest").

setup.py

@@ -263,7 +263,7 @@ setup(name=APPNAME,
                 'allmydata.util',
                 'allmydata.web',
                 'allmydata.windows',
-                'buildtest'],
+                ],
       classifiers=trove_classifiers,
       test_suite="allmydata.test",
       install_requires=install_requires,