Fix thread leaks in the thread pool

While testing an OpenCL driver with ThreadSanitizer enabled the
OpenCL-CTS suffers from thread leaks in conversions and bruteforce on
posix systems. This is because `pthread_join` is never called in
`ThreadPool_Exit` for the `pthread_t`s created by the thread pool.
Instead, the threads are only informed to stop waiting on the condition
variable which unblocks the worker thread but does not clean up after
itself.

```
ThreadPool: thread 1 exiting.
ThreadPool: thread 5 exiting.
ThreadPool: thread 4 exiting.
ThreadPool: thread 2 exiting.
ThreadPool: thread 7 exiting.
ThreadPool: thread 0 exiting.
ThreadPool: thread 3 exiting.
ThreadPool: thread 6 exiting.
Thread pool exited in a orderly fashion.
==================
WARNING: ThreadSanitizer: thread leak (pid=2292842)
  Thread T9 (tid=2292855, finished) created by main thread at:
    #0 pthread_create ../../../../src/libsanitizer/tsan/tsan_interceptors_posix.cpp:969 (libtsan.so.0+0x5ad75)
    KhronosGroup#1 ThreadPool_Init() <null> (test_conversions+0x35b2c)
    KhronosGroup#2 pthread_once ../../../../src/libsanitizer/tsan/tsan_interceptors_posix.cpp:1449 (libtsan.so.0+0x4057c)
    KhronosGroup#3 GetThreadCount() <null> (test_conversions+0x36262)
    KhronosGroup#4 DoTest(_cl_device_id*, Type, Type, SaturationMode, RoundingMode, _MTdata*) [clone .isra.0] <null> (test_conversions+0x10555)
    KhronosGroup#5 test_conversions(_cl_device_id*, _cl_context*, _cl_command_queue*, int) <null> (test_conversions+0x13226)
    KhronosGroup#6 callSingleTestFunction(test_definition, _cl_device_id*, int, int, unsigned long) <null> (test_conversions+0x2e66d)
    KhronosGroup#7 parseAndCallCommandLineTests(int, char const**, _cl_device_id*, int, test_definition*, int, unsigned long, int) <null> (test_conversions+0x2fb3a)
    KhronosGroup#8 runTestHarnessWithCheck(int, char const**, int, test_definition*, int, unsigned long, test_status (*)(_cl_device_id*)) <null> (test_conversions+0x349d8)
    KhronosGroup#9 main <null> (test_conversions+0xd725)

  And 7 more similar thread leaks.

SUMMARY: ThreadSanitizer: thread leak (OpenCL-CTS/buildbin/conversions/test_conversions+0x35b2c) in ThreadPool_Init()
```

This patch adds global state to keep track of the `pthread_t`s created
by `pthread_create` in `ThreadPool_Init`. The list of `pthread_t`s is
then used by `ThreadPool_Exit` to call `pthread_join` to cleanup the
`pthread_t`s correctly.

A near identical example, and additional explanation, can be found on
[stackoverflow](https://stackoverflow.com/questions/72435574/thread-leak-detected-when-using-condition-variable-instead-of-join-with-pthrea).

On the Windows path, a similar change is not necessary because
`_beginthread` is used which automatically cleans up after itself when
the worker thread function returns.

test_common/harness/ThreadPool.cpp

@@ -23,6 +23,7 @@
 // or any other POSIX system
 
 #include <atomic>
+#include <vector>
 
 #if defined(_WIN32)
 #include <windows.h>
@@ -58,6 +59,12 @@ CRITICAL_SECTION gAtomicLock;
 pthread_mutex_t gAtomicLock;
 #endif
 
+#if !defined(_WIN32)
+// Keep track of pthread_t's created in ThreadPool_Init() so they can be joined
+// in ThreadPool_Exit() and avoid thread leaks.
+static std::vector<pthread_t> pthreads;
+#endif
+
 // Atomic add operator with mem barrier.  Mem barrier needed to protect state
 // modified by the worker functions.
 cl_int ThreadPool_AtomicAdd(volatile cl_int *a, cl_int b)
@@ -642,6 +649,9 @@ void ThreadPool_Init(void)
             gThreadCount = i;
             break;
         }
+#if !defined(_WIN32)
+        pthreads.push_back(tid);
+#endif // !_WIN32
     }
 
     atexit(ThreadPool_Exit);
@@ -721,7 +731,20 @@ void ThreadPool_Exit(void)
                   "still active.\n",
                   gThreadCount.load());
     else
+    {
+#if !defined(_WIN32)
+        for (pthread_t pthread : pthreads)
+        {
+            if (int err = pthread_join(pthread, nullptr))
+            {
+                log_error("Error from %d from pthread_join. Unable to join "
+                          "work threads. ThreadPool_Exit failed.\n",
+                          err);
+            }
+        }
+#endif
         log_info("Thread pool exited in a orderly fashion.\n");
+    }
 }