build: Enabling -Werror in debug mode and fixing discovered issues

Change-Id: I29685dcb9240a3e0c8d7eaf28c8bc2ae720e55a1
diff --git a/tests/mgmt/fib-manager.cpp b/tests/mgmt/fib-manager.cpp
index c230497..f388f47 100644
--- a/tests/mgmt/fib-manager.cpp
+++ b/tests/mgmt/fib-manager.cpp
@@ -21,7 +21,7 @@
 
 NFD_LOG_INIT("FibManagerTest");
 
-class FibManagerFixture : protected BaseFixture, public FibEnumerationPublisherFixture
+class FibManagerFixture : public FibEnumerationPublisherFixture
 {
 public: