[qpack] add unit tests for soft errors
[qpack] add unit tests for soft errors

h2o/h2o,cwyang/h2o,yannick/h2o,cwyang/h2o,devnexen/h2o,cwyang/h2o,yannick/h2o,devnexen/h2o,h2o/h2o,cwyang/h2o,h2o/h2o,h2o/h2o,devnexen/h2o,devnexen/h2o,h2o/h2o,devnexen/h2o,yannick/h2o,h2o/h2o,devnexen/h2o,h2o/h2o,yannick/h2o,yannick/h2o,cwyang/h2o,yannick/h2o,cwyang/h2o,yannick/h2o,h2o/h2o,devnexen/h2o,yannick/h2o,h2o/h2o,devnexen/h2o,cwyang/h2o,devnexen/h2o,yannick/h2o,cwyang/h2o