Extend the capabilities of is_os to detect more platforms such as
freebsd and openbsd. Also remove `iswin()` helper function as it can be
replaced by `is_os("win")`.
Problem:
Subdirectories like "visual", "insert", "normal" encourage people to
separate *related* tests for no good reason. Typically the _mode_ is
not the relevant topic of a test (and when it is, _then_ create
an appropriate describe() or it()).
Solution:
- Delete the various `test/functional/<mode>/` subdirectories, move
their tests to more meaningful topics.
- Rename `…/normal/` to `…/editor/`.
- Move or merge `…/visual/*` and `…/insert/*` tests into here where
appropriate.
- Rename `…/eval/` to `…/vimscript/`.
- Move `…/viml/*` into here also.
* test(reorg): insert/* => editor/mode_insert_spec.lua
* test(reorg): cmdline/* => editor/mode_cmdline_spec.lua
* test(reorg): eval core tests => eval_spec.lua
