[test] define all unit tests in ot namespace (#9617)

This commit updates unit test modules to be defined under the `ot`
namespace. This aligns all the unit tests to follow the same
model, eliminating the need to use `ot::` prefix in unit test
code.

tests/unit/test_trickle_timer.cpp

@@ -34,7 +34,9 @@
 #include "common/trickle_timer.hpp"
 #include "instance/instance.hpp"
 
-static ot::Instance *sInstance;
+namespace ot {
+
+static Instance *sInstance;
 
 static uint32_t sNow = 0;
 static uint32_t sAlarmTime;
@@ -54,8 +56,6 @@ uint32_t otPlatAlarmMilliGetNow(void) { return sNow; }
 
 } // extern "C"
 
-namespace ot {
-
 void AdvanceTime(uint32_t aDuration)
 {
     uint32_t time = sNow + aDuration;