patch 9.0.0078: star register is unexpectedly changed when deleting

Problem:    Star register is changed when deleting and both "unnamed" and
            "unnamedplus" are in 'clipboard'.
Solution:   Make the use of the star register work as documented. (Ernie Rael,
            closes #10669)
diff --git a/src/testdir/check.vim b/src/testdir/check.vim
index d64e845..c4dca0b 100644
--- a/src/testdir/check.vim
+++ b/src/testdir/check.vim
@@ -233,6 +233,15 @@
   endif
 endfunc
 
+" Command to check that there are two clipboards
+command CheckTwoClipboards call CheckTwoClipboards()
+func CheckTwoClipboards()
+  " avoid changing the clipboard here, only X11 supports both
+  if !has('X11')
+    throw 'Skipped: requires two clipboards'
+  endif
+endfunc
+
 " Command to check for satisfying any of the conditions.
 " e.g. CheckAnyOf Feature:bsd Feature:sun Linux
 command -nargs=+ CheckAnyOf call CheckAnyOf(<f-args>)