[roll pigweed] pw_unit_test: Add missing return type annotations to EventHandler

The EventHandler abstract class (and LoggingEventHandler, derived from
it) was missing return-type annotations for each of its methods.

Original-Reviewed-on: https://pigweed-review.googlesource.com/c/pigweed/pigweed/+/103763

https://pigweed.googlesource.com/pigweed/pigweed
pigweed Rolled-Commits: 7e9a8db46b93aaa..eb00223b63ac677
Roller-URL: https://ci.chromium.org/b/8807359903294764305
Cq-Cl-Tag: roller-builder:pigweed-zephyr-roller
Cq-Cl-Tag: roller-bid:8807359903294764305
CQ-Do-Not-Cancel-Tryjobs: true
Change-Id: I4a4db4322f427476dba9569e3a44fadf1be406ec
Reviewed-on: https://pigweed-review.googlesource.com/c/pigweed/zephyr-integration/+/103823
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/pigweed b/pigweed
index 7e9a8db..eb00223 160000
--- a/pigweed
+++ b/pigweed
@@ -1 +1 @@
-Subproject commit 7e9a8db46b93aaa787c664ac750fbd57247430ec
+Subproject commit eb00223b63ac677f5d2e54229da70d15389738e9