Fix path to launch firefox on macOS

[koji-1009]

Fix firefox startup path, as dart test -p firefox can no longer be executed on macOS.

close #2194

I checked the workflow and both macos and firefox are removed from the execution environment. In relation to PR, do I need to add macos and firefox execution patterns to the workflow?

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[jakemac53]

Ah crap, I realize now that this doesn't work for the executables which aren't actually a file path, but instead are an executable name expected to be on your PATH.

Not sure the best way to deal with those, cc @natebosch ideas?

[natebosch]

I don't recall a way to check for an executable on the $PATH in Dart.

If macOS is the only one which needs multiple options today, should we only update the mac behavior to support multiple full paths?

[koji-1009]

We can do a workaround in dart_test.yaml, so why not change the default to firefox and have firefox-bin set in an environment that supports the old Firefox?
As Firefox browsers are updated, the importance of firefox-bin seems to gradually decrease.

We are able to do a workaround by setting the following yaml.

override_platforms:
  firefox:
    settings:
      executable: 
        mac_os: '/Applications/Firefox.app/Contents/MacOS/firefox-bin'