Check LSP capabilities before using them

test/lsp/test_lsp_command_formatting.vader

@@ -23,15 +23,14 @@ Execute(Command formatting should be applied correctly for LSP linters):
   \   'executable': has('win32') ? 'cmd': 'true',
   \   'command': '%e --foo',
   \ },
-  \ {->0}
   \)
 
   if has('win32')
     AssertEqual
-    \ ['cmd', 'cmd /s/c "cmd --foo"', '/foo/bar'],
-    \ g:args[:2]
+    \ ['cmd', 'cmd /s/c "cmd --foo"', {}],
+    \ g:args
   else
     AssertEqual
-    \ ['true', [&shell, '-c', '''true'' --foo'], '/foo/bar'],
-    \ g:args[:2]
+    \ ['true', [&shell, '-c', '''true'' --foo'], {}],
+    \ g:args
   endif