Support for running tests via new code lens

[axelson]

https://github.com/elixir-lsp/elixir-ls/pull/389 added support to build up a command to run exunit tests from a code lens (all the user has to do is click on the code lens). It would be great to be able to support this in some way in coc-elixir. The VSCode implementation is at https://github.com/elixir-lsp/vscode-elixir-ls/pull/155

Some thoughts:

• I'm not sure how interacting with the terminal will work for coc-elixir
• It might make sense to update ElixirLS to build a full example test command instead of building the test command within each extension