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
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