Fix `test_requires_ext_decorator_with_all_ext_enabled`

This test raised a wrong `TestCase.skipException`. The problem was that
the config flag `compute-feature-enabled.api_extensions` is a
`cfg.ListOpt` and as such, it expects a `list`, not a `str`.

Change-Id: I4937d024c7349f9e76d5f632852448ab5ce56576
1 file changed