build: enable full typescript checking of test files

[justinmk]

Problem

currently, test files are not checked by npm run build, we exclude them in tsconfig: https://github.com/neovim/node-client/blob/d3bc8d36b50ccc188e083b33ff537819e661dd88/packages/neovim/tsconfig.json#L7

Expected behavior

• test files should be checked
• fix all the errors