Don't overwrite existing key mappings

This makes sure that pre-existing key mappings like `<leader>t` aren't overridden.

Also adds dedicated commands for simpler mappings, e.g.

```vim
nnoremap <leader>t :RunTestFile<cr>
nnoremap <leader>T :RunNearestTest<cr>
nnoremap <leader>a :RunAllTests<cr>
```