testutil: add AtomicCounter() as a threadsafe counter

The existing code in the reqs counting is not really thread safe,
this commit fixes that.

osbuild/testutil/atomic.py

@@ -0,0 +1,29 @@
+#!/usr/bin/python3
+"""
+thread/atomic related utilities
+"""
+import threading
+
+
+class AtomicCounter:
+    """ A thread-safe counter """
+
+    def __init__(self, count: int = 0) -> None:
+        self._count = count
+        self._lock = threading.Lock()
+
+    def inc(self) -> None:
+        """ increase the count """
+        with self._lock:
+            self._count += 1
+
+    def dec(self) -> None:
+        """ decrease the count """
+        with self._lock:
+            self._count -= 1
+
+    @property
+    def count(self) -> int:
+        """ get the current count """
+        with self._lock:
+            return self._count