Fix #757 - Show :ALEDetail messages in a window

test/test_cursor_warnings.vader

@@ -97,6 +97,11 @@ After:
   " carried over between test cases.
   echomsg ''
 
+  " Close the preview window if it's open.
+  if &filetype is# 'ale-preview'
+    noautocmd :q!
+  endif
+
 Given javascript(A Javscript file with warnings/errors):
   var x = 3 + 12345678
   var x = 5*2 + parseInt("10");
@@ -138,20 +143,21 @@ Execute(The message at the cursor should be shown on InsertLeave):
 Execute(ALEDetail should print 'detail' attributes):
   call cursor(1, 1)
 
-  redir => g:output
-    ALEDetail
-  redir END
+  ALEDetail
 
-  AssertEqual "\nEvery statement should end with a semicolon\nsecond line", g:output
+  AssertEqual
+  \ ['Every statement should end with a semicolon', 'second line'],
+  \ getline(1, '$')
 
 Execute(ALEDetail should print regular 'text' attributes):
   call cursor(2, 10)
 
-  redir => g:output
-    ALEDetail
-  redir END
+  ALEDetail
 
-  AssertEqual "\nInfix operators must be spaced.", g:output
+  " ALEDetail opens a window, so check the text in it.
+  AssertEqual
+  \ ['Infix operators must be spaced.'],
+  \ getline(1, '$')
 
 Execute(ALEDetail should not capitlise cursor messages):
   call cursor(3, 1)