refatctor(nvim): reduce exptectaion for keymappings

neovim ships with default keybindings that may change per verison. the
tests, prior to this change, included these default key mappings which
make the tests fragile with each release. keeping a list of default
commands should be outside the scope of this library.

this change removes assertions against the entire set of keymaps
returned by `nvim_get_keymap` and only verifies the desired keymap has
been added and then removed.

Author: gigamaax