Use a function to define shellcheck linters and vars needed, fixes tests

ale_linters/sh/shellcheck.vim

@@ -1,10 +1,4 @@
 " Author: w0rp <devw0rp@gmail.com>
-" Description: This file adds support for using the shellcheck linter with
-"   shell scripts.
+" Description: shellcheck linter for shell scripts.
 
-call ale#linter#Define('sh', {
-\   'name': 'shellcheck',
-\   'executable':  function('ale#handlers#shellcheck#GetExecutable'),
-\   'command': function('ale#handlers#shellcheck#GetCommand'),
-\   'callback': 'ale#handlers#shellcheck#Handle',
-\})
+call ale#handlers#shellcheck#DefineLinter('sh')