Stop treating warnings as errors in tests

While this may have been well-intended, years of builds have
demonstrated that we only really see deprecation warnings in our
dependencies and rarely catch anything in colcon packages.

We may elect to re-enable this flag in our CI builds, but having it
enabled in the package itself only makes it more difficult to maintain
colcon packages downstream.

Author: cottsay

setup.cfg

@@ -51,7 +51,6 @@ test =
 
 [tool:pytest]
 filterwarnings =
-    error
     # Suppress deprecation warnings in other packages
     ignore:lib2to3 package is deprecated::scspell
     ignore:pkg_resources is deprecated as an API::flake8_import_order