Run full test suite when building from a release tag

When jenkins notices a tag has been pushed or modified (from git push -f --tags) it should check if the changelog at the tagged commit has its first entry != UNRELEASED. Then we know that that is a commit/tag the we are releasing, so it should add a build from that exact tag. And in the test suite it should run all tests (i.e. drop do not do --tag ~fragile@).

This will be a step towards involving Jenkins in our release process for real, and the full test suite run will gather some interesting data about how well the @fragile tagged tests are running on Jenkins. We could save these (ideally Jenkins should be told to keep builds of release tags longer than other builds) and look at them once or twice a year to see if it seems some tests are not fragile on Jenkins any more.

Original created by @anonym on 12391 (Redmine)