Simplify the sandbox check, to save on execution time

autoload/ale/util.vim

@@ -80,13 +80,11 @@ endfunction
 " See :help sandbox
 function! ale#util#InSandbox() abort
     try
-        call setbufvar('%', '', '')
+        function! s:SandboxCheck() abort
+        endfunction
     catch /^Vim\%((\a\+)\)\=:E48/
         " E48 is the sandbox error.
         return 1
-    catch
-        " If we're not in a sandbox, we'll get another error about an
-        " invalid buffer variable name.
     endtry
 
     return 0

test/test_sandbox_execution.vader

@@ -27,12 +27,18 @@ After:
   delfunction TestCallback
   call ale#linter#Reset()
   let g:ale_buffer_info = {}
+  unlet! b:in_sandbox
 
 Given foobar (Some imaginary filetype):
   foo
   bar
   baz
 
+Execute(ale#util#InSandbox should return 1 when in a sandbox):
+  sandbox let b:in_sandbox = ale#util#InSandbox()
+
+  Assert b:in_sandbox, 'ale#util#InSandbox() returned 0 for a sandbox command'
+
 Execute(ALE shouldn't blow up when run from a sandbox):
   AssertEqual 'foobar', &filetype