Clean up code coverage reporting.

CTest is now run directly and `make coverage` (like `make test`) now
expects you to run `make` beforehand, which is more flexible for the
user. This patch also reduces clutter by properly excluding unwanted
files and reduces the number of explicit exlusion regexes that are
required.

Gcov reports are still confusing and report very low branch coverage
(which is picked up by Codecov, unfortunately), but the llvm-cov reports
are nice and clean now.

codecov.yaml

@@ -2,5 +2,7 @@ coverage:
   range: 60..80
   round: nearest
   precision: 2
+fixes:
+  - "*/src/::"
 comment:
   require_changes: true