cubicweb #2869481 [testlib] contextual display on page validation error is broken [validation pending]

the context is calculated using position information relative to a preprocessed content, not the original source

prioritynormal
typebug
done in3.17.1
load0.250
load left0.000
closed by#045e449617ad [testlib] repreprocess content so contextual display has a chance to show the proper line. Closes #2869481
patch[testlib] repreprocess content so contextual display has a chance to show the proper line. Closes #2869481 [applied]