add tests for ale_shell option

doc/ale.txt

@@ -1990,7 +1990,7 @@ g:ale_windows_node_executable_path         *g:ale_windows_node_executable_path*
   scripts are executed with whatever executable is configured with this
   setting.
 
-g:ale_shell                                                      *g:ale_shell*
+g:ale_shell                                                       *g:ale_shell*
 
   Type: |String|
   Default: `'&shell'`
@@ -2000,7 +2000,7 @@ g:ale_shell                                                      *g:ale_shell*
   which corresponds to the $SHELL environment variable. For example 
   if `$SHELL == '/bin/bash'`, but you want to use zsh, set `g:ale_shell = '/bin/zsh'.`
 
-g:ale_shell_arguments                                  *g:ale_shell_arguments*
+g:ale_shell_arguments                                   *g:ale_shell_arguments*
 
   Type: |String|
   Default: `'&shellcmdflag'`

test/fix/test_ale_fix.vader

@@ -26,7 +26,7 @@ Before:
   augroup END
 
   if !has('win32')
-    let &shell = '/bin/bash'
+    let g:ale_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', [&shell, '-c', '''true'' --foo']],
+    \ ['true', [g:ale_shell, '-c', '''true'' --foo']],
     \ g:args
   endif

test/test_prepare_command.vader

@@ -4,6 +4,7 @@ Before:
 
 After:
   Restore
+  let g:ale_shell = &shell
 
 Execute(sh should be used when the shell is fish):
   if !has('win32')
@@ -43,6 +44,7 @@ Execute(Other shells should be used when set):
   if !has('win32')
     let &shell = '/bin/bash'
     let &shellcmdflag = '-c'
+    let g:ale_shell = &shell
 
     AssertEqual ['/bin/bash', '-c', 'foobar'], ale#job#PrepareCommand(bufnr(''), 'foobar')
   endif
@@ -54,3 +56,8 @@ Execute(cmd /s/c as a string should be used on Windows):
 
     AssertEqual 'cmd /s/c "foobar"', ale#job#PrepareCommand(bufnr(''), 'foobar')
   endif
+
+Execute(Setting ale_shell should cause ale#job#PrepareCommand to use set shell):
+  let g:ale_shell = '/foo/bar'
+
+  AssertEqual ['/foo/bar', '-c', 'foobar'], ale#job#PrepareCommand(bufnr(''), "foobar")