Refactor nix-unit tests to avoid re-evaluating the flake per test file

[jmmaloney4]

Problem

Several nix-unit integration tests re-import tests/test-flake-helpers.nix, which in turn re-evaluates tests/flake.nix and reconstructs the flake/module graph via flake-parts.lib.mkFlake.

That creates avoidable repeated evaluation work across the test suite. In the current CI profile, the detector step is already spending most of its time in flake evaluation / cache probing, so the duplicated test harness eval makes the overall situation worse.

Evidence

• tests/flake.nix instantiates the test harness flake.
• Multiple test files import tests/test-flake-helpers.nix instead of sharing one evaluated fixture.
• The slow CI job nix / ☃️ detect flake outputs spent almost all of its runtime in compute build-needed matrix via nix-eval-jobs.

Proposed fix

Refactor the test harness so the expensive flake evaluation happens once and is reused by the integration tests.

Likely shape:

• move the shared evaluated test fixture to one place
• pass that fixture into the integration tests
• keep pure/unit tests separate so they do not pay the harness cost
• remove any now-redundant nested flake evaluation path

Acceptance criteria

• The integration tests no longer re-instantiate the flake separately per file.
• The shared test fixture is evaluated once and reused.
• The affected tests still pass.
• CI evaluation time improves measurably, or at least the refactor clearly reduces duplicated evaluation in the test harness.

Notes

This is a performance and test-structure cleanup, not a behavior change.