Support loading test cases from IO stream

[LinuxSuRen]

The test case files are might stored in a HTTP server. It would be good to support loading it from an URL in this case. See also the following example usage:

atest run -p http://foo/test.yaml

See also https://github.com/LinuxSuRen/api-testing/blob/d564f69c9c0bee75fa3437be4c7cc5a3fa8c4683/pkg/testing/loader_file.go#L29