patch 7.4.1727

Problem:    Cannot detect a crash in tests when caused by garbagecollect().
Solution:   Add garbagecollect_for_testing().  Do not free a job if is still
            useful.

src/testdir/runtest.vim

@@ -60,6 +60,9 @@ let $HOME = '/does/not/exist'
 
 let s:srcdir = expand('%:p:h:h')
 
+" Prepare for calling garbagecollect_for_testing().
+let v:testing = 1
+
 " Support function: get the alloc ID by name.
 function GetAllocId(name)
   exe 'split ' . s:srcdir . '/alloc.h'