Address race in cpse_incref_decref test

[jaydoane]

Overview

Occasionally, this test fails with the following stack trace:

      cpse_gather: make_test_fun (cpse_incref_decref)...*failed*
in function cpse_test_ref_counting:'-cpse_incref_decref/1-fun-0-'/2 (src/cpse_test_ref_counting.erl, line 44)
in call from cpse_test_ref_counting:cpse_incref_decref/1 (src/cpse_test_ref_counting.erl, line 44)
in call from eunit_test:run_testfun/1 (eunit_test.erl, line 71)
in call from eunit_proc:run_test/1 (eunit_proc.erl, line 522)
in call from eunit_proc:with_timeout/3 (eunit_proc.erl, line 347)
in call from eunit_proc:handle_test/2 (eunit_proc.erl, line 505)
in call from eunit_proc:tests_inorder/3 (eunit_proc.erl, line 447)
in call from eunit_proc:with_timeout/3 (eunit_proc.erl, line 337)
**error:{assert,[{module,cpse_test_ref_counting},
         {line,44},
         {expression,"lists : member ( Pid , Pids1 )"},
         {expected,true},
         {value,false}]}
  output:<<"">>

Wrap the former assertion in a test_util:wait call to account for the apparent race between client readiness and couch_db_engine:monitored_by/1.

Testing recommendations

Related Issues or Pull Requests

Checklist

• [x] Code is written and works correctly
• [x] Changes are covered by tests
• [ ] Any new configurable parameters are documented in rel/overlay/etc/default.ini
• [ ] A PR for documentation changes has been made in https://github.com/apache/couchdb-documentation