Add new workflow to test dart-dio samples #3

Job	Run time
test (windows-latest, 3.1, samples/openapi3/client/petstore/dart-dio/petstore_client_lib_fake-jso...	16s
test (ubuntu-latest, 2.25.0, samples/openapi3/client/petstore/dart-dio/petstore_client_lib_fake-j...	11s
test (windows-latest, 2.25.0, samples/openapi3/client/petstore/dart-dio/petstore_client_lib_fake-...	16s
test (ubuntu-latest, 3.1, samples/openapi3/client/petstore/dart-dio/petstore_client_lib_fake-json...	13s
	56s

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