Remove nested namespace in pw_unit_test

This CL removes a nested namespace in pw_unit_test's framework.cc
file to maintain C++11 compatibility.

Change-Id: I0f8e477e3593d866290322ef751bfb06bcf74c24
Reviewed-on: https://pigweed-review.googlesource.com/c/pigweed/pigweed/+/15761
Reviewed-by: Wyatt Hepler <hepler@google.com>
Commit-Queue: Wyatt Hepler <hepler@google.com>
diff --git a/pw_unit_test/framework.cc b/pw_unit_test/framework.cc
index 1a27eda..5b7975f 100644
--- a/pw_unit_test/framework.cc
+++ b/pw_unit_test/framework.cc
@@ -16,7 +16,8 @@
 
 #include <cstring>
 
-namespace pw::unit_test {
+namespace pw {
+namespace unit_test {
 
 void RegisterEventHandler(EventHandler* event_handler) {
   internal::Framework::Get().RegisterEventHandler(event_handler);
@@ -121,4 +122,5 @@
 }
 
 }  // namespace internal
-}  // namespace pw::unit_test
+}  // namespace unit_test
+}  // namespace pw