patch 8.2.1979: "term_opencmd" option of term_start() is truncated

Problem:    "term_opencmd" option of term_start() is truncated. (Sergey
            Vlasov)
Solution:   Allocate the buffer to hold the command. (closes #7284)
diff --git a/src/testdir/test_terminal.vim b/src/testdir/test_terminal.vim
index ece8120..16ad720 100644
--- a/src/testdir/test_terminal.vim
+++ b/src/testdir/test_terminal.vim
@@ -567,10 +567,12 @@
   call assert_fails("call term_start(cmd, {'term_opencmd': 'split %d and %s'})", 'E475:')
   call assert_fails("call term_start(cmd, {'term_opencmd': 'split % and %d'})", 'E475:')
 
-  call term_start(cmd, {'term_finish': 'open', 'term_opencmd': '4split | buffer %d'})
+  call term_start(cmd, {'term_finish': 'open', 'term_opencmd': '4split | buffer %d | let g:result = "opened the buffer in a window"'})
   close!
   call WaitForAssert({-> assert_equal(2, winnr('$'))}, waittime)
   call assert_equal(4, winheight(0))
+  call assert_equal('opened the buffer in a window', g:result)
+  unlet g:result
   bwipe
 endfunc