Since #65043, `gpui/test-support` is safe to enable in every build, and
downstream projects (Delta) do so to avoid compiling GPUI twice. One
exception remained: with the feature on, `debug_selector` allocates its
`String` on every element construction, and paint copies it into the
frame's debug-bounds map every frame, even in release builds. Its docs
already promised it was a no-op in release.

Selectors are now recorded only in GPUI's own tests and in debug builds
with `test-support`. `VisualTestContext::debug_bounds` returns `None` in
optimized test builds; Zed doesn't run tests that way.

Release Notes:

- [GPUI] Fixed `debug_selector` allocating and recording bounds every
frame in release builds with `test-support` enabled