Add an option to run tests agaings specific braches, set "test-main-cpptango" to always test agaings heads

Now, when we prepare for tango10 and have several MRs, which can work only with particular branches of tango-idl, cpptango and tango-test, I found it usefull that individual branches can be tested against particular branches of sources.

And, in addition, I do not remember what was the logic, but I found it quite strange, that job named "test-main-cpptango" does not really test agings main branch of cpptango, but instead just install cpptango from conda (i.e. just repeat what test-source does). So I propose this job always tests against heads of tango-idl, cpptango and tango-test