test: fix flaky test-dgram-exclusive-implicit-bind

test-dgram-exclusive-implicit-bind is written assuming that dgram
messages are received with 100% reliability. While missing a dgram
message sent to localhost is rare, we do see it as evidenced by CI
failures from time to time.

The test has been rewritten to send dgram messages over and over until
the test requirements have been met.

Additional incidental refactoring includes:

* var -> const
* use of common.mustCall() instead of exit listener + boolean

PR-URL: https://github.com/nodejs/node/pull/10212


Reviewed-By: Santiago Gimeno <santiago.gimeno@gmail.com>
Reviewed-By: Italo A. Casas <me@italoacasas.com>