tests: Ensure input to subprocess is flushed

Our `memray attach` processes send a message to a subprocess and then
block waiting for it to perform some action. We need to ensure that the
message is flushed, or we will deadlock.

Signed-off-by: Matt Wozniski <mwozniski@bloomberg.net>

tests/integration/test_attach.py

@@ -121,7 +121,7 @@ def run_process(cmd, wait_for_stderr=False):
             print(exc.output)
             raise
     finally:
-        tracked_process.stdin.write("1\n")
+        print("1", file=tracked_process.stdin, flush=True)
         if wait_for_stderr:
             process_stderr = tracked_process.stderr.readline()
             while "WARNING" in process_stderr: