turbo-tasks-backend: fix transient root test race (#101700)
## Summary
Fixes a race in
`parentless_calls_of_transient_tasks_are_only_roots_if_root_functions`
(added in #101614).
The test created the transient `root` operation and made a parentless
call to `transient_endpoint` in the same `tt.run` closure, then asserted
that the endpoint had exactly one transient ref. The operation has no
output yet, so it is scheduled right away, and `tt.run` doesn't wait for
it to finish. When it runs, it calls the same `transient_endpoint` task
(`TransientValue` arguments compare by value). `connect_children` then
adds a second transient ref for that transient parent edge. Whenever the
operation finished before the assertion ran, the test saw `left: 2,
right: 1`.
The backend behavior is correct, since both refs are legitimate. The fix
only reorders the test: it makes and checks the parentless endpoint call
in its own `tt.run` before the operation exists, then creates the
operation and checks that it is a root. All the original assertions are
kept.
## Verification
- Original test under CPU load on 8 cores: 98/300 and 91/300 runs failed
at `437:5` with `left: 2`, matching the canary CI failure.
- Fixed test: 0/300 failures idle and 0/1000 under the same CPU load.
- Diagnostic A/B (not committed): waiting for the operation to complete
before the original assertion fails every time with `left: 2`. The fixed
test passes with the same wait.
- `cargo test -p turbo-tasks-backend --test
existing_outputs_from_new_roots`: 4 passed.
- `cargo clippy -p turbo-tasks-backend --test
existing_outputs_from_new_roots -- -D warnings` passes.
<!-- NEXT_JS_LLM -->
<!-- fleet 34cec176-22f1-48d8-bee2-848abef58cb3 -->
Co-authored-by: vercel-fleet-prod[bot] <318278635+vercel-fleet-prod[bot]@users.noreply.github.com>
Co-authored-by: Tobias Koppers <1365881+sokra@users.noreply.github.com>
