patch 8.2.0062: memory test is flaky on FreeBSD

Problem:    Memory test is flaky on FreeBSD.
Solution:   Add a short sleep before getting the first size.
diff --git a/src/testdir/test_memory_usage.vim b/src/testdir/test_memory_usage.vim
index d7c7036..1e0dd6d 100644
--- a/src/testdir/test_memory_usage.vim
+++ b/src/testdir/test_memory_usage.vim
@@ -133,6 +133,8 @@
 
   let vim = s:vim_new()
   call vim.start('--clean', '-c', 'set noswapfile', testfile)
+  " Wait a bit until the process has started and sourced the script.
+  sleep 200m
   let before = s:monitor_memory_usage(vim.pid).last
 
   call term_sendkeys(vim.buf, ":so %\<CR>")