docs: what to do when a test fails

[Tryanks]

Changed behaviour

Generalizes the rule the gpui-fast switch (#555) established for dependency problems to any failing test.

• Principle 8 gains: a test is truth only for a contract real use needs; a failing test is first a question about the test (its contract, and whether it drives the code as the app does) and only then a verdict on the code; such a test is deleted with the code that existed only for it; a test is never edited into passing.
• Process → When a test fails: a four-step order — reproduce as the app would (driver faults are fixed in the driver, which is what build(deps): run on gpui-fast instead of the gpui-pre snapshots #555 was), judge the assertion against a real contract and delete test plus test-only code when there is none, only then treat the code as broken, and say in the PR which it was with evidence. Applies to intermittent failures too; "make it pass" (loosen, rewrite to the new output, retry, add a seam) is named as not a step.
• Cross-references from "Tests earn their maintenance" and the AGENTS.md pointer.

Documentation only. Principle text is maintainer-authored: left open for the maintainer's review of the wording.