patch 8.1.2371: FEAT_TEXT_PROP is a confusing name

Problem:    FEAT_TEXT_PROP is a confusing name.
Solution:   Use FEAT_PROP_POPUP. (Naruhiko Nishino, closes #5291)
diff --git a/src/testdir/runtest.vim b/src/testdir/runtest.vim
index 9372d53..f0cc1ab 100644
--- a/src/testdir/runtest.vim
+++ b/src/testdir/runtest.vim
@@ -177,7 +177,7 @@
   au SwapExists * call HandleSwapExists()
 
   " Close any stray popup windows
-  if has('textprop')
+  if has('popupwin')
     call popup_clear()
   endif