DOC: regtest: make it clearer what the purpose of the "broken" series is

The purpose of the "broken" series of reg tests is to integrate scripts
which are known for triggering bugs that are not fixed at the time the
script is merged. These ones are not useful to validate non-regression
after merging a change, but have an important value to help fix the bug
they trigger. This patch updates the description in the Makefile to make
this clearer.
1 file changed