Add self-contained unit tests for sgraph (13 tests covering classification, metadata, push/pop, assoc hash, navigation)

Co-authored-by: NikolajBjorner <3085284+NikolajBjorner@users.noreply.github.com>

src/test/CMakeLists.txt

@@ -51,6 +51,7 @@ add_executable(test-z3
   escaped.cpp
   euf_bv_plugin.cpp
   euf_arith_plugin.cpp
+  euf_sgraph.cpp
   euf_seq_plugin.cpp
   ex.cpp
   expr_rand.cpp