Tests: Enable ConfigSources test on every configuration

Revise the test itself to work in all configurations and verify that
certain sources are only built by whatever configuration is tested.

Tests/ConfigSources/main.cpp

@@ -1,7 +0,0 @@
-
-#include "iface_debug.h"
-
-int main(int argc, char** argv)
-{
-  return iface_src() + iface_debug();
-}