change ale to use value of g:ale_shell regardless of what it is

test/fix/test_ale_fix.vader

@@ -26,7 +26,7 @@ Before:
   augroup END
 
   if !has('win32')
-    let g:ale_shell = '/bin/bash'
+    let &shell = '/bin/bash'
   endif
 
   call ale#test#SetDirectory('/testplugin/test')

test/lsp/test_lsp_command_formatting.vader

@@ -31,6 +31,6 @@ Execute(Command formatting should be applied correctly for LSP linters):
     \ g:args
   else
     AssertEqual
-    \ ['true', [g:ale_shell, '-c', '''true'' --foo']],
+    \ ['true', [&shell, '-c', '''true'' --foo']],
     \ g:args
   endif

test/test_prepare_command.vader

@@ -4,7 +4,7 @@ Before:
 
 After:
   Restore
-  let g:ale_shell = &shell
+  let g:ale_shell = v:null
 
 Execute(sh should be used when the shell is fish):
   if !has('win32')