Unit tests need some more consistent way of starting host

Some unit tests start host multiple times after various configuration steps which may be hard to manage properly without some workarounds in code. We should try to have more consistent and easier to maintain way of starting host. See https://github.com/apache/mynewt-nimble/pull/279 for an example of issue.

Compare the unit tests that start the host after configuration steps, using pull request #279 as the example referenced in the issue. Identify the inconsistent startup patterns and define a maintainable common approach; done means the affected tests start the host consistently without ad hoc workarounds.

Written by the indexing model from the issue text.