tests: Modify tests that print repr of an exception with 1 arg.

In Python 3.7 the behaviour of repr() of an exception with one argument
changed: it no longer prints a trailing comma in the argument list.  See
https://bugs.python.org/issue30399

This patch modifies tests that rely on this behaviour to not rely on it.
And the python34.py test is updated to include a test for this behaviour
with a .exp file.

tests/basics/python34.py.exp

@@ -11,3 +11,4 @@ SyntaxError
 SyntaxError
 3.4
 3 4
+IndexError('foo',)