Certain tests could break if you ran them separately from other tests.
They have been patched.
`run-tests` now has a `--fast` option which runs tests with only the
fastest Vim version ALE tests with, and the custom checks.
* Use sign-group only on supported vim versions.
The sign-group feature is only available in nvim 0.4.0 and vim 8.1.614.
* Add priority to ALE signs.
This allows users to set a priority to ALE signs to take precedence over
other plugin signs.
A new function is added here which will later be modified for public use
in linter and fixer callbacks. All linting and fixing now goes through
this new function, to prove that it works in all cases.
