Add new test report tool

[matuella]

Now that we've opted to not use codecov anymore (due to some bugs and some security concerns), but we still think that a tool to report the test coverage is useful - because it may indicate the health of a particular application and help while evaluating a particular PR.

We are looking for codecov alternatives, maybe using lcov or another tool that also supports Dart/Flutter?

I'm still not quite sure about the decision on codecov, totally open to discussion on this matter.