change test module importing behaviour to append to sys.path

instead of prepending.  This better allows to run test modules
against installated versions of a package even if the package
under test has the same import root.  In this example::

   testing/__init__.py
   testing/test_pkg_under_test.py
   pkg_under_test/

the tests will preferrably run against the installed version
of pkg_under_test whereas before they would always pick
up the local version.

--HG--
branch : prefer_installed

testing/python/collect.py

@@ -1,3 +1,5 @@
+import sys
+from textwrap import dedent
 import pytest, py
 
 class TestModule:
@@ -23,6 +25,24 @@ class TestModule:
             "*HINT*",
         ])
 
+    def test_import_appends_for_import(self, testdir, monkeypatch):
+        syspath = list(sys.path)
+        monkeypatch.setattr(sys, "path", syspath)
+        root1 = testdir.mkdir("root1")
+        root2 = testdir.mkdir("root2")
+        root1.ensure("x456.py")
+        root2.ensure("x456.py")
+        p = root2.join("test_x456.py")
+        p.write(dedent("""\
+            import x456
+            def test():
+                assert x456.__file__.startswith(%r)
+        """ % str(root1)))
+        syspath.insert(0, str(root1))
+        with root2.as_cwd():
+            reprec = testdir.inline_run()
+        reprec.assertoutcome(passed=1)
+
     def test_syntax_error_in_module(self, testdir):
         modcol = testdir.getmodulecol("this is a syntax error")
         pytest.raises(modcol.CollectError, modcol.collect)

testing/python/integration.py

@@ -238,7 +238,7 @@ def test_pytestconfig_is_session_scoped():
 
 
 class TestNoselikeTestAttribute:
-    def test_module(self, testdir):
+    def test_module_with_global_test(self, testdir):
         testdir.makepyfile("""
             __test__ = False
             def test_hello():
@@ -248,7 +248,7 @@ class TestNoselikeTestAttribute:
         assert not reprec.getfailedcollections()
         calls = reprec.getreports("pytest_runtest_logreport")
         assert not calls
-        
+
     def test_class_and_method(self, testdir):
         testdir.makepyfile("""
             __test__ = True