patch 8.2.5100: memory usage tests are not retried

Problem:    Memory usage tests are not retried.
Solution:   Mark memory usage tests as flaky.

src/testdir/test_memory_usage.vim

@@ -65,6 +65,9 @@ func s:term_vim.start(...) abort
   let self.job = term_getjob(self.buf)
   call WaitFor({-> job_status(self.job) ==# 'run'})
   let self.pid = job_info(self.job).process
+
+  " running an external command may fail once in a while
+  let g:test_is_flaky = 1
 endfunc
 
 func s:term_vim.stop() abort

src/version.c

@@ -734,6 +734,8 @@ static char *(features[]) =
 
 static int included_patches[] =
 {   /* Add new patch number below this line */
+/**/
+    5100,
 /**/
     5099,
 /**/