Improved regression tests for invariant contract

- Moved comments from .c files to descriptions in .desc files
- Added new tests to check multiple loops, nested loops, loop-local variables
- Enabled and clarified a regression test that was flagged as a buggy one

Author: SaswatPadhi

regression/contracts/invar_check_01/main.c

@@ -1,7 +1,3 @@
-// invar_check_02
-
-// This test checks that loop invariants adequately handle continues.
-
 #include <assert.h>
 
 int main()
@@ -11,17 +7,17 @@ int main()
 
   while(r > 0)
     __CPROVER_loop_invariant(r >= 0)
-  { 
-    --r;
-    if (r < 5)
-    {
-      continue;
-    }
-    else
     {
       --r;
+      if(r <= 1)
+      {
+        break;
+      }
+      else
+      {
+        --r;
+      }
     }
-  }
 
   assert(r == 0);
 

regression/contracts/invar_check_01/test.desc

@@ -0,0 +1,14 @@
+CORE
+main.c
+--enforce-all-contracts
+^EXIT=10$
+^SIGNAL=0$
+^\[main.1\] .* Check loop invariant before entry: SUCCESS$
+^\[main.2\] .* Check that loop invariant is preserved: SUCCESS$
+^\[main.assertion.1\] .* assertion r == 0: FAILURE$
+^VERIFICATION FAILED$
+--
+This test is expected to fail along the code path where r is an even integer
+before entering the loop.
+The program is simply incompatible with the desired property in this case --
+there is no loop invariant that can establish the required assertion.

regression/contracts/invar_check_02/test.desc

@@ -0,0 +1,25 @@
+#include <assert.h>
+
+int main()
+{
+  int r;
+  __CPROVER_assume(r >= 0);
+
+  while(r > 0)
+    __CPROVER_loop_invariant(r >= 0)
+    {
+      --r;
+      if(r <= 1)
+      {
+        break;
+      }
+      else
+      {
+        --r;
+      }
+    }
+
+  assert(r == 0 || r == 1);
+
+  return 0;
+}

regression/contracts/invar_check_03/test.desc

@@ -0,0 +1,11 @@
+CORE
+main.c
+--enforce-all-contracts
+^EXIT=0$
+^SIGNAL=0$
+^\[main.1\] .* Check loop invariant before entry: SUCCESS$
+^\[main.2\] .* Check that loop invariant is preserved: SUCCESS$
+^\[main.assertion.1\] .* assertion r == 0 || r == 1: SUCCESS$
+^VERIFICATION SUCCESSFUL$
+--
+This test checks that conditionals and `break` are correctly handled.