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/test_popupwin_textprop.vim b/src/testdir/test_popupwin_textprop.vim
index b112755..1b339d4 100644
--- a/src/testdir/test_popupwin_textprop.vim
+++ b/src/testdir/test_popupwin_textprop.vim
@@ -1,6 +1,7 @@
 " Tests for popup windows for text properties
 
 source check.vim
+CheckFeature popupwin
 CheckFeature textprop
 
 source screendump.vim