[roll third_party/pigweed] pw_allocator: Split up test utilities

This CL refactors the allocator_testing.* files and moves several types
to their own dedicated files.

The only non-trivial change is that `WithBuffer` has been moved from the
`pw::allocator::test` namespace to simply the `pw::allocator` one, as it
maybe useful for downstream implementers.

Original-Reviewed-on: https://pigweed-review.googlesource.com/c/pigweed/pigweed/+/185295

https://pigweed.googlesource.com/pigweed/pigweed
third_party/pigweed Rolled-Commits: 803032d682f869a..5dc9955f36aff34
Roller-URL: https://ci.chromium.org/b/8761341484664203825
GitWatcher: ignore
CQ-Do-Not-Cancel-Tryjobs: true
Change-Id: I60a8c5576bc1d997f8d277090a29d825f404dadb
Reviewed-on: https://pigweed-review.googlesource.com/c/pigweed/experimental/+/185455
Commit-Queue: Pigweed Roller <pigweed-roller@pigweed-service-accounts.iam.gserviceaccount.com>
Bot-Commit: Pigweed Roller <pigweed-roller@pigweed-service-accounts.iam.gserviceaccount.com>
diff --git a/third_party/pigweed b/third_party/pigweed
index 803032d..5dc9955 160000
--- a/third_party/pigweed
+++ b/third_party/pigweed
@@ -1 +1 @@
-Subproject commit 803032d682f869aff01707886eac3df27e26d58a
+Subproject commit 5dc9955f36aff3485725d378c6b14fdfdfb6813c