ci: Enalbe tests with no platform specified

After `.platform` mechanism was introduced in #159, it silently filters
out tests with no `platform` specified. Enable them to execute on
`x86_64` platform under such circumstances.

Signed-off-by: Ruoqing He <[email protected]>

.buildkite/autogenerate_pipeline.py

@@ -321,10 +321,10 @@ def build(self, input, platform_allowlist):
  if test_name in tests_to_skip:
  continue
 
- # The platform is optional. When it is not specified, we don't add
- # it to the step so that we can run the test in any environment.
+ # The platform is optional. When it is not specified, it defaults
+ # to `x86_64` to get checked against `platform_allowlist`.
  if not platforms:
- platforms = [None]
+ platforms = ["x86_64"]
 
  for platform in platforms:
  # Filter test enabled in platform_allowlist