more unique filename for test

I actually had a filename named /tmp/foo and this test failed, because
it was owned by a different user, hence writing to it (and deleting it)
failed. Using /tmp/foo_iptest makes it a bit more unique, and a lot less
likely to be on a given system.

IPython/testing/plugin/dtexample.py

@@ -45,12 +45,12 b' def ipfunc():'
     In [1]: !echo hello
     hello
 
-    In [2]: !echo hello > /tmp/foo
+    In [2]: !echo hello > /tmp/foo_iptest
 
-    In [3]: !cat /tmp/foo
+    In [3]: !cat /tmp/foo_iptest
     hello
 
-    In [4]: rm -f /tmp/foo
+    In [4]: rm -f /tmp/foo_iptest
 
     It's OK to use '_' for the last result, but do NOT try to use IPython's
     numbered history of _NN outputs, since those won't exist under the