Combine our databases into a single one

Currently we read db1 and db2 from the log and sequence files respectively.
Then we compute a cross_check database with all the tests, which have run,
as well as a would_not_run database with all the tests sets, which have not
run.

Combine those databases into a unified cross_check database and get rid of
would_not_run. We create artificial test entries for the dropped tests
sets.
While at it, we keep the spurious run tests, if any, and mark them as such.

The benefits are that we do not need special handling for dropped tests
sets down the pipe anymore, and that our future configuration file
filtering rules will also apply to spurious and dropped tests.

Signed-off-by: Vincent Stehlé <vincent.stehle@arm.com>