Handle test dependencies that are python extensions (#6904)

See #6794

This fixes cases where test-only dependencies were omitted from
consideration when modifying the environment at build time. This
includes an update to the python package definition to add
testing-related python extensions to its specialized environment
setup.

var/spack/repos/builtin/packages/python/package.py

@@ -509,7 +509,7 @@ def setup_dependent_environment(self, spack_env, run_env, dependent_spec):
 
         python_paths = []
         for d in dependent_spec.traverse(
-                deptype=('build', 'run'), deptype_query='run'):
+                deptype=('build', 'run', 'test')):
             if d.package.extends(self.spec):
                 python_paths.append(join_path(d.prefix,
                                               self.site_packages_dir))