tests/test-gendoc-sv.t

tests: explicitly define compression engines for tests

The zstd compression engine requires C extensions and isn't present
in pure Python builds.

The compression engine list leaks into the server capabilities string.

Unless we're testing functionality specific to a compression format,
the set of compression formats supported by a server doesn't matter
much.

So this commit explicitly defines the server's compression engines for
some tests so behavior is consistent between pure and non-pure builds.

Differential Revision: https://phab.mercurial-scm.org/D3431

#require docutils gettext

  $ $TESTDIR/check-gendoc sv
  checking for parse errors