patch 8.1.1581: shared functions for testing are disorganised

Problem:    Shared functions for testing are disorganised.
Solution:   Group finctions in script files. (Ozaki Kiichi, closes #4573)
diff --git a/src/testdir/test_suspend.vim b/src/testdir/test_suspend.vim
index efda68d..8b326f5 100644
--- a/src/testdir/test_suspend.vim
+++ b/src/testdir/test_suspend.vim
@@ -1,6 +1,7 @@
 " Test :suspend
 
 source shared.vim
+source term_util.vim
 
 func CheckSuspended(buf, fileExists)
   call WaitForAssert({-> assert_match('[$#] $', term_getline(a:buf, '.'))})
@@ -53,7 +54,7 @@
   " Quit gracefully to dump coverage information.
   call term_sendkeys(buf, ":qall!\<CR>")
   call term_wait(buf)
-  call Stop_shell_in_terminal(buf)
+  call StopShellInTerminal(buf)
 
   exe buf . 'bwipe!'
   call delete('Xfoo')