CMake: Simplify generation of coverage reports

There are now explicit targets for generating coverage reports from
CMake itself:

- `coverage-zerocount`: Capture initial zero coverage data and write it to
  `${COVERAGE_ZEROCOUNT_TRACEFILE}`.
- `coverage-capture`: Capture coverage data from existing .gcda files and
  write it to `${COVERAGE_CAPTURE_TRACEFILE}`.
- `coverage-unified`: Merge any coverage data files found in
  `COVERAGE_OUTPUT_DIR` and generate a unified coverage trace.
- `coverage-summary`: Print a summary of the coverage data found in
  `${COVERAGE_UNIFIED_TRACEFILE}`.
- `coverage-html_report`: Write a HTML report from the coverage data
  found in `${COVERAGE_UNIFIED_TRACEFILE}`.
- `coverage-cobertura`: Write a Cobertura report from the coverage data
  found in `${COVERAGE_UNIFIED_TRACEFILE}`.