[roll third_party/pigweed] pw_thread_freertos: Fix peak stack usage capture size

Corrects the logic for the captured peak stack usage by the thread
iteration facade in pw_thread_freertos and adds an associated test to
verify correctness. uxTaskGetStackHighWaterMark() returns remaining free
space in words rather than bytes, so the returned value needs to be
multiplied by sizeof(StackType_t) to get the remaining unused stack
space in bytes.

Original-Reviewed-on: https://pigweed-review.googlesource.com/c/pigweed/pigweed/+/110971

https://pigweed.googlesource.com/pigweed/pigweed
third_party/pigweed Rolled-Commits: e2549f9fd4a61ec..df6f1d5e7e918c0
Roller-URL: https://ci.chromium.org/b/8801925019493720369
GitWatcher: ignore
CQ-Do-Not-Cancel-Tryjobs: true
Change-Id: I4ca09f2431a2f5d8a9be5e9337ca36f2eb120a3f
Reviewed-on: https://pigweed-review.googlesource.com/c/pigweed/experimental/+/112132
Commit-Queue: Pigweed Integration Roller <pigweed-integration-roller@pigweed.google.com.iam.gserviceaccount.com>
Bot-Commit: Pigweed Integration Roller <pigweed-integration-roller@pigweed.google.com.iam.gserviceaccount.com>
diff --git a/third_party/pigweed b/third_party/pigweed
index e2549f9..df6f1d5 160000
--- a/third_party/pigweed
+++ b/third_party/pigweed
@@ -1 +1 @@
-Subproject commit e2549f9fd4a61ecd6264f1cedfe63eea709d0eb6
+Subproject commit df6f1d5e7e918c019c4ba56fee7c133f795ec4f2