The `RooRealIntegral` class is smart enough to figure out which
variables the function the integrated function doesn't depend on and
trivially integrates them itself by multiplying with the variable
definition range.

However, if the integration range is a subrange of the variable range,
this was not considered. This resulted in wrong results. for integrals
like `pdf.createIntegral(x, "subrange")`, where the pdf doesn't depend
on x. These kind of integrals can occur in the projections that the
RooAddPdf does, so it's important that they work, and fixing this
partially addresses #11486.

This change also fixes a so-far unknown bug in the `RooXYChi2Var`, which
also used these kind of integrals. Without this fix, the `Integrate()`
feature for `chi2FitTo()` was completely broken, which can be seen in
the output of the `rf609` tutorial with any previous ROOT version. The
tutorial looks okay by chance, because the function is dominted by the
quadratic term in `x` that is constant in the fit. But if one makes this
a floating parameter, the problem gets obvious.

Probably that was the reason why the main model parameter was set
constant to begin with, to sweep the bug under the rug. Now, the
tutorials are updated to have the quadratic coefficient floating too.
And also `stressRooFit`, since the reference file has to be updated
anyway.

To demonstrate that things work correctly now, a new unit test was
implemented that does the closure check of the `integrate()` feature of
the `RooXYChi2Var` with a linear fit function.