[roll third_party/pigweed] pw_rpc: Expand pw_rpc callback_client unit tests

Original-Reviewed-on: https://pigweed-review.googlesource.com/c/pigweed/pigweed/+/55023

third_party/pigweed Rolled-Commits: 9b54e5ea631e5b2..b472896bb136b08
Roller-URL: https://ci.chromium.org/b/8839959229695302736
Cq-Cl-Tag: roller-builder:pigweed-experimental-roller
Cq-Cl-Tag: roller-bid:8839959229695302736
CQ-Do-Not-Cancel-Tryjobs: true
Change-Id: I4ad2186d9208367e5055c34130d5f15451667c01
Reviewed-on: https://pigweed-review.googlesource.com/c/pigweed/experimental/+/55846
Bot-Commit: Pigweed Integration Roller <pigweed-integration-roller@pigweed.google.com.iam.gserviceaccount.com>
Commit-Queue: Pigweed Integration Roller <pigweed-integration-roller@pigweed.google.com.iam.gserviceaccount.com>
diff --git a/third_party/pigweed b/third_party/pigweed
index 9b54e5e..b472896 160000
--- a/third_party/pigweed
+++ b/third_party/pigweed
@@ -1 +1 @@
-Subproject commit 9b54e5ea631e5b2759c86952f5efdc86d00dddc8
+Subproject commit b472896bb136b08003a7bad36b87c0dd29809c6a