[tests] Test whether sseclient.EventSource works as expected
[tests] Test whether sseclient.EventSource works as expected

As found in sseclient 0.0.24 the EventSource gives randomly a
ValueError 'Unterminated string' when json.load is processed
if the limit is high enough.

Bug: T222885
Change-Id: I082a63180eb26b02bb19b53b12490bd137e322a5

wikimedia/pywikibot-core,wikimedia/pywikibot-core,PersianWikipedia/pywikibot-core