refactor(test): de-duplicate MemoryMonitor.stop() implementation

We open-coded MemoryMonitor.stop() inside __exit__. Stop doing that.

Signed-off-by: Patrick Roy <roypat@amazon.co.uk>

Author: roypat

tests/host_tools/memory.py

@@ -174,7 +174,5 @@ def __enter__(self):
     def __exit__(self, _type, _value, _traceback):
         """Exit context"""
 
-        if self.is_alive():
-            self.signal_stop()
-            self.join(timeout=1)
+        self.stop()
         self.check_samples()