Use flushdb, not flushall, in specs

What does this MR do?

Some specs need a clean Redis state to run successfully. We want to add per-environment isolation using redis databases, but to do that, we need to use flushdb instead of flushall.

This combines with gitlab-development-kit!2047 (merged) to give development/test isolation in GDK. It's safe to merge the two MRs in any order.

Without both these MRs, running certain specs in the test suite logs you out of your GDK session, and cache entries set in development can be loaded by a test, which is all sorts of fun.

Does this MR meet the acceptance criteria?

Conformity

• I have included a changelog entry, or it's not needed. (Does this MR need a changelog?)
• I have added/updated documentation, or it's not needed. (Is documentation required?)
• I have properly separated EE content from FOSS, or this MR is FOSS only. (Where should EE code go?)
• I have added information for database reviewers in the MR description, or it's not needed. (Does this MR have database related changes?)
• I have self-reviewed this MR per code review guidelines.
• This MR does not harm performance, or I have asked a reviewer to help assess the performance impact. (Merge request performance guidelines)
• I have followed the style guides.
• This change is backwards compatible across updates, or this does not apply.

Availability and Testing

• I have added/updated tests following the Testing Guide, or it's not needed. (Consider all test levels. See the Test Planning Process.)
• I have tested this MR in all supported browsers, or it's not needed.
• I have informed the Infrastructure department of a default or new setting change per definition of done, or it's not needed.

Related to gitlab-development-kit#818 (closed)