Add tests covering error handling

None of our existing tests covers the failure paths of type checking
saturating arithmetic and character literal conversion, respectively.

Author: tautschnig

regression/ansi-c/character_literals1/unsupported-wide.desc

@@ -0,0 +1,11 @@
+CORE
+unsupported.c
+-DWIDE
+wide literals with 5 characters are not supported$
+^EXIT=70$
+^SIGNAL=0$
+--
+^warning: ignoring
+^Invariant check failed$
+--
+Test to confirm that an actionable error message is provided.

regression/ansi-c/character_literals1/unsupported.c

@@ -0,0 +1,8 @@
+int main()
+{
+#ifndef WIDE
+  'abcde';
+#else
+  (void)L'abcde';
+#endif
+}

regression/ansi-c/character_literals1/unsupported.desc

@@ -0,0 +1,11 @@
+CORE
+unsupported.c
+
+literals with 5 characters are not supported$
+^EXIT=70$
+^SIGNAL=0$
+--
+^warning: ignoring
+^Invariant check failed$
+--
+Test to confirm that an actionable error message is provided.

regression/cbmc/saturating_arithmetric/typeconflict.c

@@ -0,0 +1,4 @@
+int main()
+{
+  __CPROVER_saturating_minus(1);
+}

regression/cbmc/saturating_arithmetric/typeconflict.desc

@@ -0,0 +1,7 @@
+CORE
+typeconflict.c
+file typeconflict.c line 3 function main: __CPROVER_saturating_minus takes exactly two arguments, but 1 were provided
+^EXIT=6$
+^SIGNAL=0$
+--
+^warning: ignoring