Add clarification to tests

Author: qinheping

regression/contracts-dfcc/dont_skip_cprover_prefixed_vars_fail/main.c

@@ -1,5 +1,8 @@
 void foo()
 {
+  // Initialize them with `nondet_int` to avoid them being ignored by DFCC.
+  // DFCC ignores variables that are not read/written to outside the loop
+  // or in the loop contracts.
   int nondet_var = nondet_int();
   int __VERIFIER_var = nondet_int();
   int __CPROVER_var = nondet_int();

regression/contracts-dfcc/dont_skip_cprover_prefixed_vars_pass/main.c

@@ -1,5 +1,8 @@
 void foo()
 {
+  // Initialize them with `nondet_int` to avoid them being ignored by DFCC.
+  // DFCC ignores variables that are not read/written to outside the loop
+  // or in the loop contracts.
   int nondet_var = nondet_int();
   int __VERIFIER_var = nondet_int();
   int __CPROVER_var = nondet_int();