tika: create integration tests for new server versions

We should have a set of integration tests to make sure `tika.Client` works with new server versions. Came up in #8.

Could use an environment variable to indicate if the tests should run and `t.Skip` the test if not.

We know the list of supported versions (https://github.com/google/go-tika/blob/master/tika/server.go#L150), so the test can download the server, start it, then run whatever tests it needs.

Start with tika/server.go around the supported server-version list and inspect the existing tika.Client tests and server startup entry points. Define an environment-variable opt-in, then verify each supported Tika version can be downloaded, started, and exercised by the integration tests; completion means the tests skip without that variable and pass for the enabled versions.

Written by the indexing model from the issue text.