tests: make test_crash_on_closing_tmpfile_py27 more reliable

It fails reliable for me now without the fix from 9517c3a, and had
actually a wrong assertion in the end.

Ref: pytest-dev#2370

testing/test_capture.py

@@ -1403,29 +1403,38 @@ def test_capattr():
 
 
 def test_crash_on_closing_tmpfile_py27(testdir):
-    testdir.makepyfile(
+    p = testdir.makepyfile(
         """
         from __future__ import print_function
         import time
         import threading
         import sys
 
+        printing = threading.Event()
+
         def spam():
+            global printing
+
             f = sys.stderr
+            printing.set()
             while True:
                 print('.', end='', file=f)
+                f.flush()
 
         def test_silly():
+            global printing
+
             t = threading.Thread(target=spam)
             t.daemon = True
             t.start()
-            time.sleep(0.5)
 
+            while not printing.isSet():
+                time.sleep(0.01)
     """
     )
-    result = testdir.runpytest_subprocess()
+    result = testdir.runpytest_subprocess(str(p))
     assert result.ret == 0
-    assert "IOError" not in result.stdout.str()
+    assert "IOError" not in result.stderr.str()
 
 
 def test_pickling_and_unpickling_encoded_file():