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/daemon/table/fib-nexthop.cpp b/daemon/table/fib-nexthop.cpp
index 178d853..6fec93f 100644
--- a/daemon/table/fib-nexthop.cpp
+++ b/daemon/table/fib-nexthop.cpp
@@ -26,12 +26,12 @@
 }
 
 void
-NextHop::setCost(int32_t cost)
+NextHop::setCost(uint32_t cost)
 {
   m_cost = cost;
 }
 
-int32_t
+uint32_t
 NextHop::getCost() const
 {
   return m_cost;