[roll third_party/pigweed] pw_rpc: Add SendResponseIfCalled test helper

SendResponseIfCalled combines waiting for the request for particular
method to be sent and a response to it.

This CL also adds a test_helpers.h and moves thread_testing.h contents
into it to keep all helpers together.

Original-Reviewed-on: https://pigweed-review.googlesource.com/c/pigweed/pigweed/+/98620

https://pigweed.googlesource.com/pigweed/pigweed
third_party/pigweed Rolled-Commits: 4c4f6df161ce404..437727bd2baa87e
Roller-URL: https://ci.chromium.org/b/8810549611545550657
Cq-Cl-Tag: roller-builder:pigweed-experimental-roller
Cq-Cl-Tag: roller-bid:8810549611545550657
CQ-Do-Not-Cancel-Tryjobs: true
Change-Id: Ic5655a1414a5ac6863de813be4886e54823b4187
Reviewed-on: https://pigweed-review.googlesource.com/c/pigweed/experimental/+/99522
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 4c4f6df..437727b 160000
--- a/third_party/pigweed
+++ b/third_party/pigweed
@@ -1 +1 @@
-Subproject commit 4c4f6df161ce40436c2db987522c13de145d1780
+Subproject commit 437727bd2baa87ea2c8057b70fecfdafb4a00f43