patch 8.1.1573: textprop test fails if screenhots do not work

Problem:    Textprop test fails if screenhots do not work.
Solution:   Add check for screenhots working.

src/testdir/test_textprop.vim

@@ -760,6 +760,10 @@ func Test_textprop_screenshot_visual()
 endfunc
 
 func Test_textprop_after_tab()
+  if !CanRunVimInTerminal()
+    throw 'Skipped: cannot make screendumps'
+  endif
+
   let lines =<< trim END
        call setline(1, [
              \ "\txxx",

src/version.c

@@ -777,6 +777,8 @@ static char *(features[]) =
 
 static int included_patches[] =
 {   /* Add new patch number below this line */
+/**/
+    1573,
 /**/
     1572,
 /**/