tests/test-wsgicgi.t

interfaces: introduce and use a protocol class for the `charencoding` module

See f2832de2a46c for details when this was done for the `bdiff` module.

This lets us dump the hack where the `pure` implementation was imported during
the type checking phase to provide signatures for the module methods it
provides.  Now the protocol classes are starting to shine, because these methods
are provided by `pure.charencoding` and `cext.parsers`, and references to
`cffi.charencoding` and `cext.charencoding` are forwarded to them as appropriate
by the `policy` module.  But none of that matters, as long as the module
returned provides the listed methods.

The interface was copy/pasted from the `pure` module, but `jsonescapeu8fallback`
is omitted because it is accessed from the `pure` module directly when the
escaping fails in the primary module's `jsonescapeu8()`.

  $ hg init test
  $ cat >hgweb.cgi <<HGWEB
  > #!$PYTHON
  > from mercurial import demandimport; demandimport.enable()
  > from mercurial.hgweb import hgweb
  > from mercurial.hgweb import wsgicgi
  > application = hgweb(b"test", b"Empty test repository")
  > wsgicgi.launch(application)
  > HGWEB
  $ chmod 755 hgweb.cgi

Check that non-ASCII bytes roundtrip correctly.

  $ . "$TESTDIR/cgienv"
  $ PATH_INFO="/rev/$(python -c 'import sys; sys.stdout.buffer.write(b"\xe2\x80\x94")')"; export PATH_INFO
  $ QUERY_STRING="style=raw"; export QUERY_STRING
  $ "$PYTHON" ./hgweb.cgi | grep error
  error: unknown revision '\xe2\x80\x94' (esc)