[RF] Use local message level changes in stressRooFit to silence tests

This is to avoid output that doesn't indicate any problem and only makes
the test output harder to interpret.

That's how the output of test 27 was before this commit:
```
[#1] INFO:NumericIntegration -- RooRealIntegral::init(pxyz_Int[z|R]_Norm[x,y,z]_Int[y|R]) using numeric integrator RooIntegrator1D to calculate Int(y)
Test 27 : Integration over non-rectangular regions...............OK
```

test/stressRooFit_tests.h

@@ -21,6 +21,7 @@
 #include "TCanvas.h"
 #include "RooPlot.h"
 #include "RooUnitTest.h"
+#include "RooHelpers.h"
 
 using namespace RooFit ;
 
@@ -2903,17 +2904,23 @@ class TestBasic313 : public RooUnitTest
   // C a l c u l a t e   i n t e g r a l   o f   n o r m a l i z e d   p d f   i n   R
   // ----------------------------------------------------------------------------------
 
-  // Create integral over normalized pdf model over x,y,z in "R" region
-  RooAbsReal* intPdf = pxyz.createIntegral(RooArgSet(x,y,z),RooArgSet(x,y,z),"R") ;
+  {
+    // To remove the INFO:NumericIntegration ouput from the stressRooFit output,
+    // change the message level locally.
+    RooHelpers::LocalChangeMsgLevel chmsglvl{RooFit::INFO, 0u, RooFit::NumIntegration, false};
+
+    // Create integral over normalized pdf model over x,y,z in "R" region
+    RooAbsReal* intPdf = pxyz.createIntegral(RooArgSet(x,y,z),RooArgSet(x,y,z),"R") ;
 
-  // Plot value of integral as function of pdf parameter z0
-  RooPlot* frame = z0.frame(Title("Integral of pxyz over x,y,z in region R")) ;
-  intPdf->plotOn(frame) ;
+    // Plot value of integral as function of pdf parameter z0
+    RooPlot* frame = z0.frame(Title("Integral of pxyz over x,y,z in region R")) ;
+    intPdf->plotOn(frame) ;
 
 
-  regPlot(frame,"rf313_plot1") ;
+    regPlot(frame,"rf313_plot1") ;
 
-  delete intPdf ;
+    delete intPdf ;
+  }
 
   return kTRUE;
   }