Fix more random Windows test issues

test/sign/test_linting_sets_signs.vader

@@ -33,7 +33,7 @@ Before:
   call ale#linter#Define('foobar', {
   \ 'name': 'testlinter',
   \ 'callback': 'TestCallback',
-  \ 'executable': 'echo',
+  \ 'executable': has('win32') ? 'cmd' : 'echo',
   \ 'command': 'echo foo bar',
   \})
 

test/sign/test_sign_placement.vader

@@ -66,7 +66,7 @@ Before:
 
   call ale#linter#Define('testft', {
   \ 'name': 'x',
-  \ 'executable': 'true',
+  \ 'executable': has('win32') ? 'cmd' : 'true',
   \ 'command': 'true',
   \ 'callback': 'GenerateResults',
   \})