Fix should_add_user_to_db_admins test suite

[eiri]

Test should_add_user_to_db_admins is horribly unreliable. Current master from travis:

in function couch_peruser_test:'-should_add_user_to_db_admins/1-fun-0-'/0 (test/couch_peruser_test.erl, line 321)
**error:{assertEqual,[{module,couch_peruser_test},
              {line,321},
              {expression,"proplists : get_value ( << \"admins\" >> , get_security ( UserDbName ) )"},
              {expected,{[{<<"names">>,[<<"qux">>]}]}},
              {value,undefined}]}
  output:<<"">>

Expected Behavior

Test shouldn't break on a whim for unrelated changes

Current Behavior

There are obvious race in that test and it is plaguing us for longer than half-year, if one can trust search on closed PRs claiming to fix it.

Possible Solution

Find and fix that annoying race.

Steps to Reproduce (for bugs)

Dunno, it's a race. Run suite enough times to catch it? Insert timers in random parts to induce it?

[eiri]

On reproduction: timer:sleep(200) in ensure_security/3 works just perfect, so even slight fabric delay on getting that security object leads to an empty (but still 200 OK) response and the test's falure.