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.
diff --git a/src/testdir/test_textprop.vim b/src/testdir/test_textprop.vim
index 36eeb70..df060ea 100644
--- a/src/testdir/test_textprop.vim
+++ b/src/testdir/test_textprop.vim
@@ -760,6 +760,10 @@
 endfunc
 
 func Test_textprop_after_tab()
+  if !CanRunVimInTerminal()
+    throw 'Skipped: cannot make screendumps'
+  endif
+
   let lines =<< trim END
        call setline(1, [
              \ "\txxx",