Projects that depend on GPUI rebuild it, and everything above it,
whenever they switch between running their app and running their tests:
`cargo test` turns on `test-support` through dev-dependencies, and
`cargo build` doesn't. Turning `test-support` on in every build would
remove that split, but today the feature changes runtime behavior, so it
can't ship in a production binary. This PR makes it additive where it
mattered.

- **Leak detection is opt-in.** `gpui/test-support` no longer turns on
`leak-detection`, which takes the entity map's write lock on every
handle create and drop. It's now enabled by the `leak-detection` feature
(as `zed/track-project-leak` already does) or by setting
`GPUI_LEAK_DETECTION=1` when building. Without it:
  - `leak_detector_snapshot()` and `assert_no_new_leaks()` are no-ops;
- `WeakEntity::assert_released()` still panics when the entity is alive,
but can't list the live handles.
- **`util::paths::home_dir()`** returns the fixed fake path
(`/Users/zed`) only in `util`'s own tests, not whenever
`util/test-support` is on. No test outside `util` depended on the fake
path.
- **`is_test()` on GPUI contexts.** `App::is_test`, `AsyncApp::is_test`
and `BackgroundExecutor::is_test` (in gpui and scheduler) report whether
the executor is a `TestScheduler`, in every build. Code that must behave
differently under tests should use these instead of `cfg(feature =
"test-support")`, so compiling the feature into an app doesn't change
its behavior. For code with no executor, such as expensive invariant
checks, `scheduler::test_scheduler_created()` reports whether the
process has created a `TestScheduler`, which test harnesses do before
the test body runs.
- **Metal `framebuffer_only(false)`** (needed for offscreen capture) now
only applies in debug builds, so optimized builds that compile
`test-support` keep the faster presentation path.

**Open question:** CI doesn't set `GPUI_LEAK_DETECTION`, so with this
change leak checks outside gpui's own unit tests are skipped in CI. If
we go ahead, the test jobs should set it. I've left that out until we
agree on the approach.

This PR is mainly to see what CI says about crates whose tests read
`home_dir()` (`project`, `worktree`, `workspace`, …), which now see the
real home directory.

Release Notes:

- [GPUI] Made the `test-support` feature safe to enable in production
builds: leak detection is now opt-in via the `leak-detection` feature or
`GPUI_LEAK_DETECTION`, and `App::is_test` reports whether code runs
under the test scheduler