logilab-common #1716420 testlib: allow fail or skip in generative tests [validation pending]

if the fail or skip happens in the generator (rather than when executing the yielded function), we report it as an error.

priorityminor
typebug
done in1.1.0
load0.500
load left0.000
closed by#b643f601a028 [testlib] Allow skipTest() and fail() in generative tests
patch[testlib] Allow skipTest() and fail() in generative tests [applied]