Add stronger integration tests

[m-pilia]

The current test strategy uses mocks of the LSP client APIs. This provides a controlled environment and helps preventing regressions, but it is not enough to prevent issues due to breaking changes in the dependencies (e.g. #39).

The purpose of this issue is to implement a full integration test that involves the latest version of the dependencies. The rough plan is:

• in each build, download the latest version of each dependency and test against it (ALE, vim-lsp, vim-lsc, coc.nvim, etc.)
• either set-up a real, minimal project, and get/build ccls, or implement a mock of ccls