refactor(tests): organize test namespaces

tests/core/result_test.cpp

@@ -4,7 +4,7 @@
 
 #include "result.hpp"
 
-namespace {
+namespace core::tests {
 
 using namespace core;
 
@@ -108,9 +108,9 @@ void runAllTests() {
   testBasicResultWithCustomError();
 }
 
-} // namespace
+} // namespace core::tests
 
 int main() {
-  runAllTests();
+  core::tests::runAllTests();
   return 0;
 }