test(policy): update e2e_egress_network for reliability

[olix0r]

The e2e_egress_network test is failing for two reasons:

1. httpbin.org is currently down and returning 503 errors.
2. The default tests change the default policies and relaunch a container after the policy is changed. In under-resourced environments, this update may be slow or unreliable.

This change fixes this by:

1. Using postman-echo.com instead of httpbin.org.
2. Splitting the tests into distinct allow and deny tests to avoid mutating the default policy.