Backend tests should not reuse unique keys.

[mshima]

Overview of the issue

At spring-boot v2.6.0 migration, reactive mongodb tests started having lots of random backend tests failures.
Those failures look driver related, but we can fix the failures by improving our tests and stop reusing unique keys like login and emails.

See #16787.

JHipster Version(s)

main

JHipster configuration

unrelated

Entity configuration(s) entityName.json files generated in the .jhipster directory

unrelated

Browsers and Operating System

unrelated

• Checking this box is mandatory (this is just to show you read everything)

[mshima]

Looks like the underlying driver issues was fixed.
Reactive mongodb tests are passing now.