[RF] Also test different BatchModes in `createNLL` in stressRooFit

In stressRooFit, you could already globally select the BatchMode for
`fitTo` calls. With this commit, the selection is also considered for
`createNLL` calls, widening the test coverage of the new BatchMode.

In particular, this tests the plotting of likelihood functions, which is
much faster when using the new BatchMode backend (up to a factor of 10
in the first profile NLL tuutorial).