patch 8.2.0936: some terminals misinterpret the code for getting cursor style

Problem:    Some terminals misinterpret the code for getting cursor style.
Solution:   Send a sequence to the terminal and check the result. (IWAMOTO
            Kouichi, closes #2126)  Merged with current code.

src/testdir/test_quickfix.vim

@@ -2710,7 +2710,6 @@ func Test_cwindow_highlight()
   CheckScreendump
 
   let lines =<< trim END
-	set t_u7=
 	call setline(1, ['some', 'text', 'with', 'matches'])
 	write XCwindow
 	vimgrep e XCwindow