tests: resetGlobalIoService for every test

All tests must use BaseFixture or a fixture derived from it to get this feature.

This commit also fixes a few warnings in tests/mgmt, and moves test cases into nfd::tests namespace.

refs #1290

Change-Id: I891441a5abce170e35648d463f7157b18429f79f
diff --git a/tests/fw/strategy-tester.hpp b/tests/fw/strategy-tester.hpp
index f8e0848..571492c 100644
--- a/tests/fw/strategy-tester.hpp
+++ b/tests/fw/strategy-tester.hpp
@@ -11,6 +11,7 @@
 #include "fw/strategy.hpp"
 
 namespace nfd {
+namespace tests {
 
 /** \class StrategyTester
  *  \brief extends strategy S for unit testing
@@ -64,7 +65,7 @@
   onAction();
 }
 
-
+} // namespace tests
 } // namespace nfd
 
 #endif // TEST_FW_STRATEGY_TESTER_HPP