Add external identifier test to wikibase_tests.py
Add external identifier test to wikibase_tests.py

A follow-up to I7fd4f5f4bba8a4eb2f83b8b093eb2faf2e64f811 now that
there are external identifier properties on Wikidata.

Also renames variables in test_set_incorrect_target_value() to make
it clearer which type is being tested.

Change-Id: I145de5835bb600330700e1bab92daeadefe35786

npdoty/pywikibot,magul/pywikibot-core,h4ck3rm1k3/pywikibot-core,hasteur/g13bot_tools_new,hasteur/g13bot_tools_new,Darkdadaah/pywikibot-core,jayvdb/pywikibot-core,happy5214/pywikibot-core,jayvdb/pywikibot-core,h4ck3rm1k3/pywikibot-core,smalyshev/pywikibot-core,Darkdadaah/pywikibot-core,happy5214/pywikibot-core,magul/pywikibot-core,wikimedia/pywikibot-core,hasteur/g13bot_tools_new,darthbhyrava/pywikibot-local,npdoty/pywikibot,PersianWikipedia/pywikibot-core,wikimedia/pywikibot-core