[roll third_party/pigweed] pw_rpc_transport: Use enable_if for tests

Original-Reviewed-on: https://pigweed-review.googlesource.com/c/pigweed/pigweed/+/151132

https://pigweed.googlesource.com/pigweed/pigweed
third_party/pigweed Rolled-Commits: 8ab12829aba42a7..002c567f284f9f7
Roller-URL: https://ci.chromium.org/b/8778742876413959457
GitWatcher: ignore
CQ-Do-Not-Cancel-Tryjobs: true
Change-Id: I85d704443d296a1a845c2ed8066ec79620051095
Reviewed-on: https://pigweed-review.googlesource.com/c/pigweed/experimental/+/151151
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 8ab1282..002c567 160000
--- a/third_party/pigweed
+++ b/third_party/pigweed
@@ -1 +1 @@
-Subproject commit 8ab12829aba42a71430e59429862d568bfa4917b
+Subproject commit 002c567f284f9f72cc02204ecce4a53ae17c37c9