test/py: Allow tests to be filtered by role

Some test can only be run by a particular board in a lab, e.g. because
they are loaded with an OS image used by the test. Add a way to specify
this in tests.

Signed-off-by: Simon Glass <sjg@chromium.org>
diff --git a/test/py/pytest.ini b/test/py/pytest.ini
index 26d83f8..361be01 100644
--- a/test/py/pytest.ini
+++ b/test/py/pytest.ini
@@ -12,3 +12,4 @@
     requiredtool: U-Boot: Required host tools for a test.
     slow: U-Boot: Specific test will run slowly.
     singlethread: Cannot run in parallel
+    role: U-Boot: Indicates the lab 'role' which can execute this test