Fix couch peruser test suite

[eiri]

Overview

This PR removes some unreliable helper functions and addresses a race condition in acquiring security object that triggered occasionally test failures.

Testing recommendations

Local run make eunit apps=couch_peruser suites=couch_peruser_test, but most important Travis should stop breaking in couch peruser test suite.

Related Issues or Pull Requests

Resolves #1236

Checklist

• Code is written and works correctly;
• Changes are covered by tests;
• Documentation reflects the changes;

[jiangphcn]

+1 after squashing and making travis happy.

Eunit test passed locally.

======================== EUnit ========================
couch_peruser test
Application crypto was left running!
  couch_peruser_test:177: should_create_anon_user_db_with_default...[0.009 s] ok
  couch_peruser_test:178: should_create_anon_user_db_with_default...ok
  couch_peruser_test:186: should_create_anon_user_db_with_custom_prefix...[0.002 s] ok
  couch_peruser_test:193: should_create_anon_user_db_with_custom_special_prefix...[0.002 s] ok
  couch_peruser_test:153: should_create_user_db_with_default...[0.003 s] ok
  couch_peruser_test:154: should_create_user_db_with_default...ok
  couch_peruser_test:162: should_create_user_db_with_custom_prefix...[0.003 s] ok
  couch_peruser_test:169: should_create_user_db_with_custom_special_prefix...[0.002 s] ok
  couch_peruser_test:203: should_create_user_db_with_q4...[0.004 s] ok
  couch_peruser_test:204: should_create_user_db_with_q4...ok
  couch_peruser_test:215: should_create_anon_user_db_with_q4...[0.003 s] ok
  couch_peruser_test:216: should_create_anon_user_db_with_q4...ok
  couch_peruser_test:228: should_not_delete_user_db...ok
  couch_peruser_test:228: should_not_delete_user_db...ok
  couch_peruser_test:240: should_delete_user_db...ok
  couch_peruser_test:240: should_delete_user_db...ok
  couch_peruser_test:255: should_delete_user_db_with_custom_prefix...ok
  couch_peruser_test:256: should_delete_user_db_with_custom_prefix...ok
  couch_peruser_test:272: should_delete_user_db_with_custom_special_prefix...ok
  couch_peruser_test:273: should_delete_user_db_with_custom_special_prefix...ok
  couch_peruser_test:305: should_reflect_config_changes...ok
  couch_peruser_test:306: should_reflect_config_changes...ok
  couch_peruser_test:307: should_reflect_config_changes...ok
  couch_peruser_test:308: should_reflect_config_changes...ok
  couch_peruser_test:309: should_reflect_config_changes...ok
  couch_peruser_test:310: should_reflect_config_changes...ok
  couch_peruser_test:319: should_add_user_to_db_admins...[0.001 s] ok
  couch_peruser_test:328: should_add_user_to_db_members...[0.002 s] ok
  couch_peruser_test:347: should_not_remove_existing_db_admins...ok
  couch_peruser_test:348: should_not_remove_existing_db_admins...ok
  couch_peruser_test:349: should_not_remove_existing_db_admins...ok
  couch_peruser_test:367: should_not_remove_existing_db_members...ok
  couch_peruser_test:368: should_not_remove_existing_db_members...ok
  couch_peruser_test:369: should_not_remove_existing_db_members...ok
  couch_peruser_test:398: should_remove_user_from_db_admins...ok
  couch_peruser_test:399: should_remove_user_from_db_admins...ok
  couch_peruser_test:400: should_remove_user_from_db_admins...ok
  couch_peruser_test:401: should_remove_user_from_db_admins...ok
  couch_peruser_test:402: should_remove_user_from_db_admins...ok
  couch_peruser_test:403: should_remove_user_from_db_admins...ok
  couch_peruser_test:432: should_remove_user_from_db_members...ok
  couch_peruser_test:433: should_remove_user_from_db_members...ok
  couch_peruser_test:434: should_remove_user_from_db_members...ok
  couch_peruser_test:435: should_remove_user_from_db_members...ok
  couch_peruser_test:436: should_remove_user_from_db_members...ok
  couch_peruser_test:437: should_remove_user_from_db_members...ok
[os_mon] cpu supervisor port (cpu_sup): Erlang has closed
  [done in 34.851 s]
=======================================================
  All 46 tests passed.
==> rel (eunit)
==> couchdb (eunit)