cli-plugins: minor cleanups: use Println

- use Println to print newline instead of custom format
- suppress some errors to make my IDE and linters happier
- use res.Assert() with icmd.Expected{} where possible to make
  assertions not depend on newline / whitespace randomness
- use apiClient instead of client for the API client to
  prevent shadowing imports.
- use dockerCLI with Go's standard camelCase casing.

Signed-off-by: Sebastiaan van Stijn <github@gone.nl>

e2e/cli-plugins/plugins/presocket/main.go

@@ -113,7 +113,7 @@ func RootCmd(dockerCli command.Cli) *cobra.Command {
 			select {
 			case <-done:
 			case <-time.After(2 * time.Second):
-				_, _ = fmt.Fprint(dockerCli.Err(), "timeout after 2 seconds")
+				_, _ = fmt.Fprintln(dockerCli.Err(), "timeout after 2 seconds")
 			}
 			return nil
 		},