Add support for disabling test cases.

The JSON test case format has an "enabled" key that is not
currently being honored. This trivial patch adds support for skipping
test cases that has enabled set to 0.

test/regression/regression.cc

@@ -93,8 +93,11 @@ void perform_unit_test(ModSecurityTest<RegressionTest> *test,
     std::vector<RegressionTest *> *tests,
     ModSecurityTestResults<RegressionTestResult> *res, int *count) {
 
-
     for (RegressionTest *t : *tests) {
+        if (t->enabled == 0) {
+            continue;
+        }
+
         CustomDebugLog *debug_log = new CustomDebugLog();
         modsecurity::ModSecurity *modsec = NULL;
         modsecurity::Rules *modsec_rules = NULL;