add test case related to the exit function

Signed-off-by: Lucas Cordeiro <[email protected]>

Author: lucasccordeiro

regression/cbmc-slice/slice18/main.c

@@ -0,0 +1,11 @@
+void exit(int status);
+
+int main() {
+  int x;
+  
+  if(x==10)
+    exit(1);
+
+  if(x==10)
+    assert(0);
+}

regression/cbmc-slice/slice18/test.desc

@@ -0,0 +1,8 @@
+CORE
+main.c
+
+^EXIT=0$
+^SIGNAL=0$
+^VERIFICATION SUCCESSFUL$
+--
+^warning: ignoring