tests: support testing with Neovim

[laniakea64]

This adds TEST_VIM env var setting, which can be set to point to the Vim or Neovim executable to test with. If the specified executable's file_stem contains "nvim", the test runner will treat it as Neovim. Otherwise, the test runner will treat it as Vim. If this env var is not set, run vim same as before.

To avoid Neovim creating .local/state/nvim in the repo, not sure whether is better to use a tempdir for $XDG_STATE_HOME, as this patch does now, or to instead set NVIM_LOG_FILE to something like /dev/null?

Not seeing any measurable performance regression in testing with Vim.

Closes #80

Edit: Also added fix for #86 per #86 (comment)

[laniakea64]

@NoahTheDuke Sorry, forgot to mention that the reason for requesting your review before merge was not just the question about .local/state/nvim, it's for the Rust code changes in general as that was a slight stretch of my current Rust knowledge level.

[laniakea64]

Hi @NoahTheDuke might you be able to review these Rust code changes soon? Would be good to get this merged before just adds new syntaxes, there's some backlog in just PRs and past experience suggests things could move quickly at the moment casey finds time to get back to just. Thanks

[NoahTheDuke]

left two comments, otherwise it's good to go.

[NoahTheDuke]