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/unit/unit_test.h

@@ -25,6 +25,12 @@
 
 namespace modsecurity_test {
 
+class UnitTestResult {
+ public:
+    int ret;
+    std::string output;
+};
+
 class UnitTest {
  public:
     static UnitTest *from_yajl_node(const yajl_val &);
@@ -39,9 +45,8 @@ class UnitTest {
     std::string filename;
     std::string output;
     int ret;
-    int obtained;
     int skipped;
-    std::string obtainedOutput;
+    UnitTestResult result;
 };
 
 }  // namespace modsecurity_test