Reorder includes in smt_to_smt2_string unit test

Because adding an include in the following commit will cause
clang-format to re-order the includes. Doing the reorder in advance
makes it possible to review the re-ordering and the additional include
separately.

Author: thomasspriggs

unit/solvers/smt2_incremental/smt_to_smt2_string.cpp

@@ -1,15 +1,14 @@
 // Author: Diffblue Ltd.
 
-#include <testing-utils/use_catch.h>
+#include <util/mp_arith.h>
 
 #include <solvers/smt2_incremental/smt_commands.h>
 #include <solvers/smt2_incremental/smt_core_theory.h>
 #include <solvers/smt2_incremental/smt_logics.h>
 #include <solvers/smt2_incremental/smt_sorts.h>
 #include <solvers/smt2_incremental/smt_terms.h>
 #include <solvers/smt2_incremental/smt_to_smt2_string.h>
-
-#include <util/mp_arith.h>
+#include <testing-utils/use_catch.h>
 
 TEST_CASE("Test smt_indext to string conversion", "[core][smt2_incremental]")
 {