Added support to run unit tests in a multithreaded context

- This is controlled by specifying the 'mtstress' argument when running
  `unit_test`.
- The goal is to detect if the operator/transformation  fails in this
  context.
- In this mode, the test will be executed 5'000 times in 50 threads
  concurrently.
- Allocation & initialization of the operator/transformation is
  performed once in the main thread, while the evaluation is executed in
  the threads.
  - This is consistent with the library's support for multithreading,
    where initialization and loading of rules is expected to run once.
    See issue #3215.

test/common/modsecurity_test.cc

@@ -93,13 +93,13 @@ bool ModSecurityTest<T>::load_test_json(const std::string &file) {
 
 
 template <class T>
-std::pair<std::string, std::vector<T *>>*
+void
 ModSecurityTest<T>::load_tests(const std::string &path) {
     DIR *dir;
     struct dirent *ent;
     struct stat buffer;
 
-    if ((dir = opendir(path.c_str())) == NULL) {
+    if ((dir = opendir(path.c_str())) == nullptr) {
         /* if target is a file, use it as a single test. */
         if (stat(path.c_str(), &buffer) == 0) {
             if (load_test_json(path) == false) {
@@ -107,10 +107,10 @@ ModSecurityTest<T>::load_tests(const std::string &path) {
                 std::cout << std::endl;
             }
         }
-        return NULL;
+        return;
     }
 
-    while ((ent = readdir(dir)) != NULL) {
+    while ((ent = readdir(dir)) != nullptr) {
         std::string filename = ent->d_name;
         std::string json = ".json";
         if (filename.size() < json.size()
@@ -123,16 +123,15 @@ ModSecurityTest<T>::load_tests(const std::string &path) {
         }
     }
     closedir(dir);
-
-    return NULL;
 }
 
 
 template <class T>
-std::pair<std::string, std::vector<T *>>* ModSecurityTest<T>::load_tests() {
-    return load_tests(this->target);
+void ModSecurityTest<T>::load_tests() {
+    load_tests(this->target);
 }
 
+
 template <class T>
 void ModSecurityTest<T>::cmd_options(int argc, char **argv) {
     int i = 1;
@@ -144,6 +143,10 @@ void ModSecurityTest<T>::cmd_options(int argc, char **argv) {
         i++;
         m_count_all = true;
     }
+    if (argc > i && strcmp(argv[i], "mtstress") == 0) {
+        i++;
+        m_test_multithreaded = true;
+    }
     if (std::getenv("AUTOMAKE_TESTS")) {
         m_automake_output = true;
     }