fix test failure due to compilation order (see #973)

some tests rely on additional diagnostics code being linked in,
which happens, when lib/format-util.hpp is included prior to
the instantiation of lib::diff::Record rsp. lib::Variant.

The reason why i opended this can of worms was to avoid includion
of this formatting and diagnostics code into such basic headers
as lib/variant.hpp or lib/diff/gen-node.hpp

Now it turns out, that on some platforms the linker will use
a later instantiation of lib::Variant::Buff<GenNode>::operator string
in spite of a complete instantiation of this virtual function
being available already in liblumierasupport.so

But the real reason is that -- with this trickery -- we're violating
the single definition rule, so we get what we deserved.

TODO (Ticket #973): at a later point in development we have to re-assess,
the precise impact of including lib/format-util.hpp into
lib/diff/gen-node.hpp
Right now I expect GenNode to be used pervasively, so I am
reluctant to make that header too heavyweight.

src/lib/diff/gen-node.hpp

@@ -96,9 +96,7 @@
 #include "lib/error.hpp"
 #include "lib/idi/entry-id.hpp"
 #include "lib/time/timevalue.hpp"
-//#include "lib/util.hpp"
-//#include "lib/format-string.hpp"
-//#include "lib/format-util.hpp"
+//#include "lib/format-util.hpp"        ///////////////////////////////TICKET #973 : investigate the impact of this inclusion on code size
 #include "lib/diff/record.hpp"
 #include "lib/variant.hpp"
 #include "lib/util.hpp"

tests/library/diff/diff-tree-application-test.cpp

@@ -22,6 +22,7 @@
 
 
 #include "lib/test/run.hpp"
+#include "lib/format-util.hpp"
 #include "lib/diff/tree-diff-application.hpp"
 #include "lib/iter-adapter-stl.hpp"
 #include "lib/time/timevalue.hpp"