Do not alter test_config global in test_extension_ext_gw_mode

Bug 1199557

Instead, allow subclasses to instruct the base test class which
API extension manager they should use as it is already done with
the plugin name.

Change-Id: If4e53ffca3bdeb4fb218918358437aca41489de4
Merge "Do not alter test_config global in test_extension_ext_gw_mode"