test_slapgrid: support virtualenv paths

When using virtualenv, the module paths are not simple list, but
instances of importlib._bootstrap_external._NamespacePath.

According to docs [1] module's __path__ does is not a list, but an
iterator producing strings.

Cast to a list to consume these iterators.

[1]
https://docs.python.org/3/reference/import.html?highlight=__path__#path__

slapos/tests/test_slapgrid.py

@@ -69,7 +69,7 @@ from slapos.util import dumps
 from slapos import __path__ as slapos_path
 from zope import __path__ as zope_path
 
-PROMISE_PATHS = sorted(set(map(os.path.dirname, slapos_path + zope_path)))
+PROMISE_PATHS = sorted(set(map(os.path.dirname, list(slapos_path) + list(zope_path))))
 
 import httmock