Fix race condition bug in test, and show assertion failure values

[kkwoker]

Two changes:

1. Fixed a bug in the test where the expected_result was caught in a race condition with the first two tests using promises.
2. Cosmetic fixes for the test by removing the strings, making the assert return failure values.

Checklist

• make -j4 test (UNIX), or vcbuild test (Windows) passes
• tests and/or benchmarks are included
• commit message follows commit guidelines

Affected core subsystem(s)

test

[joyeecheung]

CI: https://ci.nodejs.org/job/node-test-pull-request/10445/

[refack]

Suggestion: declare promise which is now a scoped var as const.

[refack]

Except for lint error:

not ok 37 - /usr/home/iojs/build/workspace/node-test-linter/test/addons-napi/test_promise/test.js
  ---
  message: '''expected_result'' is never reassigned. Use ''const'' instead.'
  severity: error
  data:
    line: 11
    column: 7
    ruleId: prefer-const
  messages:
    - message: '''expected_result'' is never reassigned. Use ''const'' instead.'
      severity: error
      data:
        line: 24
        column: 7
        ruleId: prefer-const
  ...

CI is clean (CentOS5 failure is irrelevant)

[refack]

Welcome @kkwoker, and thank you for the contribution 🥇
Hope you find the time to follow up on this PR.

[tniessen]

Nit: I would prefer not to have single lines separated by empty lines for assertions.

[kkwoker]

Hi, thank you for your help! I made the relevant fixes from your suggestions

[refack]

@kkwoker thank you for following up.
Linter is now green - https://ci.nodejs.org/job/node-test-linter/12314/ ✔️

P.S. You had some unrelated commits from master in this PR so I rebased and force pushed. You can sync up by running git pull --rebase

[joyeecheung]

CI: https://ci.nodejs.org/job/node-test-pull-request/10480/

[refack]

Landed in 84579b1

@kkwoker congratulations on being promoted from

to

🍾