[roll third_party/pigweed] pw_unit_test: Require 'light' backend for RPC service

The pw_unit_test RPC service only supports the 'light' backend. If it's
used with another backend such as GoogleTest, there will be misleading
compilation errors and the service won't find any of the tests.

This commit adds a build assertion to check that the correct backend is
in use if the RPC service is used.

Original-Reviewed-on: https://pigweed-review.googlesource.com/c/pigweed/pigweed/+/97907

https://pigweed.googlesource.com/pigweed/pigweed
third_party/pigweed Rolled-Commits: 9a41bfeb2355143..c3bdb8abc4bd72b
Roller-URL: https://ci.chromium.org/b/8811428695371834001
Cq-Cl-Tag: roller-builder:pigweed-experimental-roller
Cq-Cl-Tag: roller-bid:8811428695371834001
CQ-Do-Not-Cancel-Tryjobs: true
Change-Id: Iab8ff190d0d3edcc33cbfc77abe670412a23768e
Reviewed-on: https://pigweed-review.googlesource.com/c/pigweed/experimental/+/97952
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 9a41bfe..c3bdb8a 160000
--- a/third_party/pigweed
+++ b/third_party/pigweed
@@ -1 +1 @@
-Subproject commit 9a41bfeb2355143d3d2fb3d8f78a9b973ba809f7
+Subproject commit c3bdb8abc4bd72b5f7e9ff8d3e9cfc1f83b9c53d