Revert no-assertions-on-coverage, or make it controlled by an option.

In 1228dd293b60a we made TOR_COVERAGE equavalent to NDEBUG so that branch coverage could be accurate.

But we hate NDEBUG.

I just spent 20 minutes too long debugging a problem because I'd forgotten that I was building with enable_coverage and that as such I shouldn't expect tor_assert to do anything.

We should either revert 1228dd293b60a, or make an --enable-branch-coverage option for configure that you have to use if you want to do branch coverage this way.