Prevent SAST analyzers from scanning test files

What does this MR do?

SAST vulnerabilities have started showing up for *_test.go files for some of our analyzers. We can ignore these.

Recent example:

https://gitlab.com/gitlab-org/security-products/analyzers/security-code-scan/-/security/vulnerabilities/19053004

I also removed the no-longer-supported SAST_DISABLE_DIND reference.

What are the relevant issue numbers?

Does this MR meet the acceptance criteria?

• Tests added for this feature/bug