cmake: Add new code coverage targets. (#6796)

cmake: Add new code coverage targets. (#6796)

This commit adds the following new code coverage targets:

- coverage-reset: Resets the code coverage counters

- coverage: Generates code coverage report for all cvc5 executions since the
            last coverage-reset

- coverage-test: This was previously the coverage target that runs the
                 tests and generates the coverage report (as used for
                 nightlies).

By using `make coverage-reset` and `make coverage` it is now possible to
generate coverage reports for arbitrary executions of cvc5.