patch 8.1.0088: terminal test for stdout and stderr is a bit flaky

Problem:    Terminal test for stdout and stderr is a bit flaky.
Solution:   Wait for both stdout and stderr to have been processed. (Ozaki
            Kiichi, closes #2991)

src/testdir/test_terminal.vim

@@ -1487,8 +1487,9 @@ func Test_terminal_out_err()
 
   let outfile = 'Xtermstdout'
   let buf = term_start(['./Xechoerrout.sh'], {'out_io': 'file', 'out_name': outfile})
-  call WaitForAssert({-> assert_inrange(1, 2, len(readfile(outfile)))})
-  call assert_equal("this is standard out", readfile(outfile)[0])
+
+  call WaitFor({-> !empty(readfile(outfile)) && !empty(term_getline(buf, 1))})
+  call assert_equal(['this is standard out'], readfile(outfile))
   call assert_equal('this is standard error', term_getline(buf, 1))
 
   call WaitForAssert({-> assert_equal('dead', job_status(term_getjob(buf)))})

src/version.c

@@ -761,6 +761,8 @@ static char *(features[]) =
 
 static int included_patches[] =
 {   /* Add new patch number below this line */
+/**/
+    88,
 /**/
     87,
 /**/