Convert the mock for gym unit tests from a Python string to an HTML file. This makes the mocking of gym unit tests more consistent with how unit tests for other modules are mocked. This also avoids unnecessarily mocking entire module functions, as only the HTTP requests made by those functions need to be mocked. Since the mock is only supposed to contain an HTML HTTP response, it makes more sense to keep it as HTML.

I placed the new HTML mock in the tests/samples/ directory rather than the tests/mocks/ directory where the current mock is located. I'm doing this as part of a larger effort to migrate all the mocks in tests/mocks/ to tests/samples so that mocks won't be split across two sibling directories.