cmd/test: Default `-v` traces show notes and fails

The default value for `--explain` was used when `--verbose`/`-v` was
specified. Now if only a verbose flag is specified we will show both
"fails" and "notes", which is most likely what someone would want
when troubleshooting some test failure.

Fixes: #2068
Signed-off-by: Patrick East <east.patrick@gmail.com>