[RF] Disable bin averaging for residHist and pullHist in stressRooFit
[RF] Disable bin averaging for residHist and pullHist in stressRooFit

The default behaviour of RooPlot::residHist/pullHist was changed in
commit c1e3740 to enable averaging of the curve values over each bin.

The reference histograms for the RooFit stress tests were produced
with the bin averaging disabled, so bin average is explicitely disabled
in these tests to match the test reference.

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