Run all tests in CI even when a snapshot test fails (#8506)

Run the tests with `--no-fail-fast`, in CI and in `scripts/check.sh`.

`cargo test` stops after the first failing test target, so one failing
snapshot test in `egui_demo_app` hid the failing snapshots of every
later crate, and it took several CI rounds to collect all the new
snapshots. Now one run reports them all and uploads all `.new.png`
files, so one kitdiff pass is enough.

`egui_kittest` already collects all snapshot errors of a test in one
`SnapshotResults` and only panics when it is dropped, so no change is
needed there.

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Co-authored-by: Claude Opus 5 (1M context) <noreply@anthropic.com>