tests/test-simplemerge.py.out

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

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OK