Use non root user as default for tests

[rimadeodhar]

Currently, we end up defaulting to using root user while running logictests and the default test cluster settings. We should switch to using non root user to allow for testing codepaths that are enforced for non root users (for e.g. system.privileges checks etc.).

This request comes from discussion during this postmortem.

Tests to consider:

• logic tests
• unit tests that use the in-memory TestServer
• roachtests

Jira issue: CRDB-24743

[blathers-crl]

cc @cockroachdb/test-eng

[rafiss]

Closing since we aren't able to prioritize this large change. Instead we'll rely on #63145 being completed for roactests, and when we write other new tests, we'll decide to use non-root users on a case-by-case basis.