Improve test cases for boolean values

In I6a3ef892ac54389d30d2e9e25b9ad8047d4926ec we see some confusion about
the order which CLI values vs config files values should be applied.

The documented semantics is that values in config files override any
values specified on the command line.

Improve the "bool value override" tests to more explicitly test the
cases of a config file overriding a CLI bool, a config file overriding
an inverse CLI bool and a config file overriding a previous config file
bool. The proposed patch would have broken these tests.

Change-Id: I133c43d2d9121c98407d42d6cab8a1ecb2ff450e