Fix tests for fixed AUTOBUILD_WIN_VSTOOLSET setting.

The test was passing AUTOBUILD_VSVER=171, expecting the previous behavior that
AUTOBUILD_WIN_VSTOOLSET would *not* be set. But that's exactly what the
previous commit is intended to fix: we want AUTOBUILD_VSVER=171 to behave like
AUTOBUILD_VSVER=170 and emit the corresponding AUTOBUILD_WIN_VSTOOLSET.

Instead, use an old AUTOBUILD_VSVER=120, which predates the table in
autobuild_tool_source_environment.py.

Author: nat-goodspeed

tests/test_source_environment.py

@@ -326,6 +326,6 @@ def test_vstoolset_set(self):
 
     @needs_cygwin
     def test_vstoolset_not_set_if_vsver_unrecognized(self):
-        with envvar("AUTOBUILD_VSVER", "171"):
+        with envvar("AUTOBUILD_VSVER", "120"):
             vars = self.read_variables(self.find_data("empty"))
         self.assertTrue("AUTOBUILD_WIN_VSTOOLSET" not in vars)