[RF] Fix `RooAddPdf::expectedEvents()` in conditional fits

[guitargeek]

So far, the projection coefficient for coefficients in RooAddPdfs was missing in RooAddPdf::expectedEvents(). However, this is important to include in conditional fits where the normalization set of expectedEvents() is different from the normalization set for coefficient determination. This PR includes this missing factor.

A unit test that covers both the case where the expected events are taken from the coefs and the one where they are taken from the pdfs is included.

[github-actions[bot]]

Test Results

    13 files      13 suites   2d 21h 41m 58s :stopwatch:  3 026 tests  3 026 :white_check_mark: 0 :zzz: 0 :x: 33 807 runs  33 807 :white_check_mark: 0 :zzz: 0 :x:

Results for commit 38aba9cc.

:recycle: This comment has been updated with latest results.