patch 8.0.1162: shared script for tests cannot be included twice

Problem:    Shared script for tests cannot be included twice.
Solution:   Include it where needed, it will "finish" if loaded again.
diff --git a/src/testdir/test_bufline.vim b/src/testdir/test_bufline.vim
index f57dd73..cc5c10e 100644
--- a/src/testdir/test_bufline.vim
+++ b/src/testdir/test_bufline.vim
@@ -1,8 +1,6 @@
 " Tests for setbufline() and getbufline()
 
-if !exists('*GetVimCommand')
-  source shared.vim
-endif
+source shared.vim
 
 func Test_setbufline_getbufline()
   new