Testing of reverse dependencies (qtdeclarative tests when testing incoming qtbase changes)

Currently, all testing in CI of a given Qt5 module will test only that module (plus the minimum compilation of its dependencies).

We should also support the testing of reverse dependencies, to verify that incoming changes to one module are not unexpectedly breaking another module.

The first and primary use-case for this feature would be to compile and autotest qtdeclarative for incoming qtbase changes. This is important because qtdeclarative depends a lot on qtbase behavior which is not covered by the qtbase compile or autotests.

Some points about the proposed implementation:

• existing configurations are not changed; there should be one or more new configurations, e.g. "revdep-qtdeclarative linux-g++-32 Ubuntu 10.04 x86"
• the revdep tests should skip if the sync.profile in the revdep module is such that the incoming change would anyway not be used in the revdep module's CI.
  • For example, if we are testing an incoming change to qtbase master, then we would do the revdep-qtdeclarative test if and only if qtdeclarative/sync.profile contains qtbase => "refs/heads/master".
  • This also is the mechanism used to allow "known" breaking changes. e.g. if you have a qtbase change which you know will break qtdeclarative, you can pin qtdeclarative to a qtbase SHA1 prior to your change, then submit your qtbase change, then submit your qtdeclarative fix/update together with a change unpinning qtbase.