Prepare some test for pytest in completers.

swap the context manager and the function definition.
It is undefined wither the test are collected and then run at once; or
ran while they are being yielded.

In the fist case (what pytest does) the function was called outside the
context manager, in the later case (node) it was working as expected.

IPython/core/tests/test_completer.py

@@ -387,8 +387,8 b' def test_greedy_completions():'
     _,c = ip.complete('.',line='a[0].')
     nt.assert_false('.real' in c,
                     "Shouldn't have completed on a[0]: %s"%c)
-    with greedy_completion(), provisionalcompleter():
         def _(line, cursor_pos, expect, message, completion):
+        with greedy_completion(), provisionalcompleter():
             ip.Completer.use_jedi = False
             _,c = ip.complete('.', line=line, cursor_pos=cursor_pos)
             nt.assert_in(expect, c, message % c)
@@ -398,6 +398,7 b' def test_greedy_completions():'
                 completions = ip.Completer.completions(line, cursor_pos)
             nt.assert_in(completion, completions)
 
+    with provisionalcompleter():
         yield _, 'a[0].', 5, 'a[0].real', "Should have completed on a[0].: %s", Completion(5,5, 'real')
         yield _, 'a[0].r', 6, 'a[0].real', "Should have completed on a[0].r: %s", Completion(5,6, 'real')