We found that Coveralls is not able to merge coverage reports from
different job runs. Since supporting PHP8, we have different code
branches in order to support the old and the new PDO interface.

So, it is crucial to merge coverage reports from two job runs, one
for PHP7.4 and another for PHP8.

This will aid on running the tests with coverage on both PHP7 and PHP8,
merge coverage reports and render them as HTML to be inspected by
humans.

It's not completely maintenance free because some paths will probably
have to be adjusted for others, but it's way better than nothing.

With the introduction of PHP8 compatibility, some `class_alias` code
has been introduced. We have not been able to make PHPUnit grok the
coverage of this code by other means.

Additionally, some annotations have been added to improve code coverage
metrics on code which actually has been covered but was not accounted.