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

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