Travis failure: users_db.js

[wohali]

Another random failure on Travis:

test/javascript/tests/users_db.js                              
    Error: The database could not be created, the file already exists.
Trace back (most recent call first):
    
 551: test/javascript/couch.js
      CouchError([object Object])
 514: test/javascript/couch.js
      ([object CouchHTTP])
  35: test/javascript/couch.js
      ()
 208: test/javascript/tests/users_db.js
      ()
 395: test/javascript/couch_test_runner.js
      run_on_modified_server([object Array],(function () {try {testFun();} f
 202: test/javascript/tests/users_db.js
      ()
  37: test/javascript/cli_runner.js
      runTest()
  48: test/javascript/cli_runner.js

Logs at https://couchdb-vm2.apache.org/ci_errorlogs/travis-couchdb-348317175-2018-03-02T17:19:49.035728/couchlog.tar.gz say:

[notice] 2018-03-02T17:19:42.647364Z node1@127.0.0.1 <0.30826.3> 1fa2310f22 127.0.0.1:15984 127.0.0.1 undefined POST /_session 200 ok 44
[notice] 2018-03-02T17:19:48.631957Z node1@127.0.0.1 <0.30826.3> b849059fd4 127.0.0.1:15984 127.0.0.1 jan DELETE /_users/?sync=true 200 ok 5984
[notice] 2018-03-02T17:19:48.634764Z node1@127.0.0.1 <0.30826.3> 5c78b0cab8 127.0.0.1:15984 127.0.0.1 jan PUT /_users/ 412 ok 1

Bad test case. We tried our best to eliminate any immediate DELETE/PUT cycles but this one slipped through the cracks.

[janl]

Closing in favour of #1441