[infra] fix flag validation in test_runner

[sigmundch]

Recently the test_runner started validating that we don't override a flag that is implied by the named configuration. The previous validation was accidentally omitted until https://dart-review.googlesource.com/c/sdk/+/333842.

This broke the macos web bots because they need to specify the arch option when running test.py. Otherwise, the sharding infrastructure failed to properly run the tests in the correct architecture.

To keep some validation, but continue to allow the bots to operate, I am temporarily skipping the check for the arch flag. Ideally we should be able to add it back, by fixing how the sharding works.

[sigmundch]

cc @athomas @nshahan @mraleph

[sigmundch]

Here is the change to temporarily allow using arch on the command line with a named configuration: https://dart-review.googlesource.com/c/sdk/+/333705

[sigmundch]

Example failure when the arch parameter is not specified on the step that needs sharding: https://ci.chromium.org/ui/p/dart/builders/try/ddc-mac-chrome-try/45/overview