The etcdctl and etcdutl built with `-tags cov` mode will show go-test result
after each execution, like
```
...
PASS
coverage: 0.0% of statements in ./...
```
Since the PASS is not real command, the `source completion` command will
fail with command-not-found error. And there is no easy way to disable
the (*testing.M).Run's output. Therefore, this patch uses build tag !cov
to disable cases when enable coverage.
Fixes: #14694
Signed-off-by: Wei Fu <fuweid89@gmail.com>
