tests: additional test coverage of archive web command This command is special in a few ways. First, it is the only command using the write() function from WSGI's start_response() function. Second, it is setting a custom content-disposition header. We change the test so it prints out full details of the HTTP response. We also save the response body to a file so we can verify its size and hash. The hash check will help ensure that archive generation is deterministic. Differential Revision: https://phab.mercurial-scm.org/D2790