Tests: Check for ModuleNotFoundError on Python 3.6+

Those tests originally checked for ImportError. Since Python 3.6
ModuleNotFoundError is raised in this context instead, the tests didn't work
as they are text based (so exception inheritance does not save the day).

Fixes https://github.com/pytest-dev/pytest/issues/2132

testing/test_doctest.py

@@ -4,6 +4,10 @@ import _pytest._code
 from _pytest.doctest import DoctestItem, DoctestModule, DoctestTextfile
 import pytest
 
+PY36 = sys.version_info[:2] >= (3, 6)
+MODULE_NOT_FOUND_ERROR = 'ModuleNotFoundError' if PY36 else 'ImportError'
+
+
 class TestDoctests:
 
     def test_collect_testtextfile(self, testdir):
@@ -211,8 +215,8 @@ class TestDoctests:
         # doctest is never executed because of error during hello.py collection
         result.stdout.fnmatch_lines([
             "*>>> import asdals*",
-            "*UNEXPECTED*ImportError*",
-            "ImportError: No module named *asdal*",
+            "*UNEXPECTED*{e}*".format(e=MODULE_NOT_FOUND_ERROR),
+            "{e}: No module named *asdal*".format(e=MODULE_NOT_FOUND_ERROR),
         ])
 
     def test_doctest_unex_importerror_with_module(self, testdir):
@@ -227,7 +231,7 @@ class TestDoctests:
         # doctest is never executed because of error during hello.py collection
         result.stdout.fnmatch_lines([
             "*ERROR collecting hello.py*",
-            "*ImportError: No module named *asdals*",
+            "*{e}: No module named *asdals*".format(e=MODULE_NOT_FOUND_ERROR),
             "*Interrupted: 1 errors during collection*",
         ])