Add the smt-backend tags to tests that use --z3

This means that the core tests will run cleanly without z3 on the path.

Author: martin

regression/cbmc/struct15/test.desc

@@ -1,4 +1,4 @@
-CORE
+CORE smt-backend
 main.c
 --trace --z3
 ^EXIT=0$

regression/cbmc/z3/invalid.desc

@@ -1,4 +1,4 @@
-CORE
+CORE smt-backend
 invalid.c
 --trace --smt2 --z3
 ^EXIT=10$

regression/cbmc/z3/valid.desc

@@ -1,4 +1,4 @@
-CORE
+CORE smt-backend
 valid.c
 --trace --smt2 --z3
 ^EXIT=0$