Failing test for #1489
Turn `plugin uninstall --no-delete` into `plugin uninstall
--skip-delete`

The `--no-delete` flag has been broken since
a9adf7b78bcefe3b5db03e167f53c6263dab1549 (5 releases), so I'm not
concerned about a backwards compat shim.

Adds more verbosity too.
Update existing test for 894a35ced5edabf4502f34758e5107732cd76b34
Merge pull request #1489 from wp-cli/1489-uninstall-skip-delete

Unknown parameter when --no-* is specified