patch 8.2.0425: code for modeless selection not sufficiently tested

Problem:    Code for modeless selection not sufficiently tested.
Solution:   Add tests.  Move mouse code functionality to a common script file.
            (Yegappan Lakshmanan, closes #5821)
diff --git a/src/testdir/test_global.vim b/src/testdir/test_global.vim
index 7804ca4..d3bfaa9 100644
--- a/src/testdir/test_global.vim
+++ b/src/testdir/test_global.vim
@@ -5,7 +5,10 @@
   set clipboard=unnamed
   g/^/normal yyp
   call assert_equal(['a', 'a', 'b', 'b', 'c', 'c'], getline(1, 6))
-
+  set clipboard=unnamed,unnamedplus
+  call setline(1, ['a', 'b', 'c'])
+  g/^/normal yyp
+  call assert_equal(['a', 'a', 'b', 'b', 'c', 'c'], getline(1, 6))
   set clipboard&
   bwipe!
 endfunc