TestFillPluginDefaults broken on 3.x Enterprise

[rainest]

3.x adds a new websockets option for protocols, but only on Enterprise. This defaults test fails because the Enterprise and OSS defaults are different:

 === RUN   TestFillPluginDefaults/no_config_no_protocols
    plugin_service_test.go:568:   &kong.Plugin{
          	... // 8 identical fields
          	RunOn:    &"test",
          	Ordering: nil,
          	Protocols: []*string{
          		... // 2 identical elements
          		&"http",
          		&"https",
        - 		&"ws",
        - 		&"wss",
          	},
          	Tags: nil,
          }
=== RUN   TestFillPluginDefaults/partial_config_no_protocols
    plugin_service_test.go:568:   &kong.Plugin{
          	... // 8 identical fields
          	RunOn:    nil,
          	Ordering: nil,
          	Protocols: []*string{
          		... // 2 identical elements
          		&"http",
          		&"https",
        - 		&"ws",
        - 		&"wss",
          	},
          	Tags: nil,
          }

The 3.x Enterprise defaults will also differ from the 2.x Enterprise defaults.

We need to create a separate 3.x Enterprise test or add version-dependent behavior internal to the test. The various skip functions don't really accommodate the former yet (you can skip Enterprise, but it skips all versions, or only allow a given Enterprise range while also skipping OSS).