Add support for running coverage report from Test UI

[lukaszsamson]

It should be possible to add

const runProfile = controller.createRunProfile(
    "Coverage",
    vscode.TestRunProfileKind.Coverage,
    (request, token) => {
      ...
    }
  );

Depends on https://github.com/elixir-lsp/vscode-elixir-ls/issues/306