platform: give the macOS HEVC probe test its own binary again

It must be the first in its process to ask `hevc_available()`, but #2860 moved it into the shared
`it` binary next to `hardware_encode`, which asks too. Under `cargo test` (xtask's fallback without
nextest) the two raced and it failed with "nothing has asked yet". Same layout as its Windows twin
`nvenc_hevc_warm.rs`.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

crates/platform/tests/hevc_probe.rs

@@ -1,6 +1,6 @@
 //! `register()` answers the HEVC availability probe in the background, so the first draw of the
-//! format list does not create a hardware session on the UI thread. One test in this file on
-//! purpose: anything else that asked `hevc_available()` first would make the wait below meaningless.
+//! format list does not create a hardware session on the UI thread. Its own test binary on purpose:
+//! anything else that asked `hevc_available()` first would make the wait below meaningless.
 #![cfg(target_os = "macos")]
 
 use std::time::{Duration, Instant};

crates/platform/tests/it/main.rs

@@ -1,7 +1,6 @@
 mod common;
 mod fallback;
 mod hardware_encode;
-mod hevc_probe;
 mod media_foundation;
 mod nvenc;
 mod nvenc_export;