Add comment and update test_06 skip message

cibuildwheel/linux.py

@@ -16,6 +16,7 @@ from .util import (
 def matches_platform(identifier):
     pm = platform.machine()
     if pm == "x86_64":
+        # x86_64 machines can run i686 docker containers
         if identifier.endswith('x86_64') or identifier.endswith('i686'):
             return True
     elif pm == "i686":

test/06_docker_images/cibuildwheel_test.py

@@ -9,9 +9,10 @@ import utils
 def test():
     project_dir = os.path.dirname(__file__)
 
-    if utils.platform != 'linux' or platform.machine() not in ['x86_64',
-                                                               'i686']:
+    if utils.platform != 'linux':
         pytest.skip('the test is only relevant to the linux build')
+    if platform.machine() not in ['x86_64', 'i686']:
+        pytest.skip('this test is currently only possible on x86_64/i686 due to availability of alternative images')
 
     actual_wheels = utils.cibuildwheel_run(project_dir, add_env={
         'CIBW_MANYLINUX_X86_64_IMAGE': 'dockcross/manylinux2010-x64',