[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_child.cpp

@@ -37,7 +37,7 @@
 
 namespace ot {
 
-static ot::Instance *sInstance;
+static Instance *sInstance;
 
 enum
 {