$TESTDIR is added to the path, so this is superfluous. Also,
inconsistent use of quotes means we might have broken on tests with
paths containing spaces.
In order to mimic module-level evaluation, globals and locals should be the
same object, so doctest does not pass separate locals dict.
https://docs.python.org/2.7/reference/simple_stmts.html#exec
This fixes NameError in the following example:
>>> import foo
>>> def bar():
... foo # must exist in globalvars
