patch 8.2.1448: test 77a for VMS depends on small.vim which does not exist

Problem:    Test 77a for VMS depends on small.vim which does not exist.
Solution:   Use the 'silent while 0" trick. (issue #6696)
diff --git a/src/testdir/test77a.in b/src/testdir/test77a.in
index 052e29a..b4f38b5 100644
--- a/src/testdir/test77a.in
+++ b/src/testdir/test77a.in
@@ -10,7 +10,11 @@
 
 VMS does not have CKSUM but has a built in CHECKSUM - it should be used
 STARTTEST
-:so small.vim
+:silent! while 0
+:  e! test.ok
+:  w! test.out
+:  qa!
+:silent! endwhile
 :if !has("vms")
 : e! test.ok
 : w! test.out