Switch off the default diagnostic provider

Remove the default testing diagnostic provider if another implementation
is found.
Exclude testing diagnostics provider

This will exclude the testing diagnostics provider from Nancy if there's
another interactive diagnostics provider available.
Revert "Switch off the default diagnostic provider"

This reverts commit 9d8f13a09d6011510151cd53960c65390a95fc2b.
Try to make the logic more readable
Add tests
Format DiagnosticsHookFixture
Merge pull request #1106 from jugglingnutcase/adjust-default-diagnostics

Switch off the default diagnostic provider