e2e/cli-plugins: explicitly check that PersistentPreRunE works

I regressed this in d4ced2ef77 ("allow plugins to have argument which match a
top-level flag.") by unconditionally overwriting any `PersistentRunE` that the
user may have supplied.

We need to ensure two things:

1. That the user can use `PersistentRunE` (or `PersistentRun`) for their own
   purposes.
2. That our initialisation always runs, even if the user has used
   `PersistentRun*`, since that will shadow the root.

To do this add a `PersistentRunE` to the helloworld plugin which logs (covers 1
above) and then use it when calling the `apiversion` subcommand (which covers 2
since that uses the client)

Signed-off-by: Ian Campbell <ijc@docker.com>

e2e/cli-plugins/testdata/docker-help-helloworld.golden

@@ -6,6 +6,7 @@ A basic Hello World plugin for tests
 Options:
   -c, --context string   Is it Christmas?
   -D, --debug            Enable debug
+      --pre-run          Log from prerun hook
       --who string       Who are we addressing?
 
 Commands: