Running tests should incrementally increase coverage rather than clearing coverage from other tests

[lampuiho]

It seems to be a result of the following issue where a user requested that running tests clear code coverage but instead it is now clearing all the coverage.

#202712

If I run each test class 1 by 1, I see that code coverage of all other unrelated code also gets cleared.
This means if I want to add tests to add more coverage, I have to re-run all the tests to see the full result before I can go to those files to see what is not covered.

I would like that it incrementally adds lines that the test covers.