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/core/limited-io.cpp b/tests/core/limited-io.cpp
index 5752685..ea19585 100644
--- a/tests/core/limited-io.cpp
+++ b/tests/core/limited-io.cpp
@@ -7,6 +7,7 @@
 #include "limited-io.hpp"
 
 namespace nfd {
+namespace tests {
 
 const int LimitedIo::UNLIMITED_OPS = std::numeric_limits<int>::max();
 const time::Duration LimitedIo::UNLIMITED_TIME = time::nanoseconds(-1);
@@ -15,7 +16,6 @@
   : m_isRunning(false)
   , m_nOpsRemaining(0)
 {
-  resetGlobalIoService();
 }
 
 LimitedIo::StopReason
@@ -55,4 +55,5 @@
   getGlobalIoService().stop();
 }
 
+} // namespace tests
 } // namespace nfd