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