[PartitionAlloc] Re-enable `PartitionAllocPreforkTest.PreforkHandler`

Re-enable `PreforkHandler` in `base_unittests` (as a non-parameterized
`PartitionAllocPreforkTest.PreforkHandler` test) by invoking
`std::_Exit(1)` instead of `exit(1)` in the death test child process,
forcing `"fast"` death test style, and guarding the test with
`PA_CONFIG(HAS_ATFORK_HANDLER)`.

In https://crrev.com/c/2696369 (tracked in https://crbug.com/40642257),
`PartitionAllocTest.PreforkHandler` was disabled because the test runner
on some platforms reported `status: UNKNOWN` despite the test passing.

In Chromium's test launcher (`base/test/launcher/`), a test is reported
as `TEST_UNKNOWN` when the test executable succeeds but the launcher
cannot parse the machine-readable XML report. When `EXPECT_EXIT()` runs
with `"fast"` death tests (`fork()` without `exec()`), the child process
inherits the open file descriptor and stdio buffer of the XML result
file. Calling `exit(1)` invokes libc `exit()` cleanup, running C++
global destructors in the child. This invoked
`XmlUnitTestResultPrinter`'s destructor, which prematurely emitted
`</testsuites>` and closed the stream before the parent process finished
writing the test outcome, corrupting the XML report and causing the
launcher to report `UNKNOWN`.

Calling `std::_Exit(1)` terminates the child process immediately
without invoking `atexit()` handlers or C++ static destructors, leaving
the XML report stream intact and allowing the test launcher to record
test success.

Additionally:
- Guard the test with `PA_CONFIG(HAS_ATFORK_HANDLER)` so it only runs on
  platforms that register PartitionAlloc's `pthread_atfork()` handlers
  (`IS_APPLE || IS_LINUX || IS_CHROMEOS`).
- Explicitly set `GTEST_FLAG_SET(death_test_style, "fast")` because
  `TestSuite::PreInitialize()` defaults to `"threadsafe"` death tests on
  non-Android platforms, which re-executes the test binary rather than
  exercising `BeforeForkInParent()`, `AfterForkInParent()`, and
  `AfterForkInChild()` in a forked child.
- Convert the test from `TEST_P(PartitionAllocTest, ...)` to
  `TEST(PartitionAllocPreforkTest, PreforkHandler)` because it only
  exercises global `malloc()` and `free()` rather than the fixture's
  parameterized `allocator`.

Verified on Linux.

Part of http://go/arthurmilchior-fix-all-test-skill-chrome for AI week.

Fixed: 40642257
Change-Id: Ia12ee2a7dad6e9412a5165d3059fd816d6d96cc7
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/8646089
Commit-Queue: Arthur Milchior <arthurmilchior@chromium.org>
Reviewed-by: Mikihito Matsuura <mikt@google.com>
Auto-Submit: Arthur Milchior <arthurmilchior@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1745147}
NOKEYCHECK=True
GitOrigin-RevId: 2cdafa96a5af94dff08b2c7757a5ad8a2860982c