patch 8.2.3518: Test_xrestore sometimes fails

Problem:    Test_xrestore sometimes fails.
Solution:   Mark the test as flayky.  Move marking test as flaky to the test
            instead of listing them in runtest.
diff --git a/src/testdir/test_channel.vim b/src/testdir/test_channel.vim
index fbe6a84..f8cdef4 100644
--- a/src/testdir/test_channel.vim
+++ b/src/testdir/test_channel.vim
@@ -795,6 +795,7 @@
 endfunc
 
 func Test_close_output_buffer()
+  let g:test_is_flaky = 1
   enew!
   let test_lines = ['one', 'two']
   call setline(1, test_lines)
@@ -935,6 +936,7 @@
 
 func Run_pipe_through_sort(all, use_buffer)
   CheckExecutable sort
+  let g:test_is_flaky = 1
 
   let options = {'out_io': 'buffer', 'out_name': 'sortout'}
   if a:use_buffer
@@ -1206,6 +1208,7 @@
 endfunc
 
 func Test_out_cb()
+  let g:test_is_flaky = 1
   let dict = {'thisis': 'dict: '}
   func dict.outHandler(chan, msg) dict
     if type(a:msg) == v:t_string
@@ -1333,6 +1336,7 @@
 endfunc
 
 func Test_close_and_exit_cb()
+  let g:test_is_flaky = 1
   let g:retdict = {'ret': {}}
   func g:retdict.close_cb(ch) dict
     let self.ret['close_cb'] = a:ch->ch_getjob()->job_status()
@@ -1560,6 +1564,7 @@
 
 func Test_exit_callback_interval()
   CheckFunction reltimefloat
+  let g:test_is_flaky = 1
 
   let g:exit_cb_val = {'start': reltime(), 'end': 0, 'process': 0}
   let job = [s:python, '-c', 'import time;time.sleep(0.5)']->job_start({'exit_cb': 'MyExitTimeCb'})
@@ -1742,6 +1747,7 @@
 endfunc
 
 func Test_collapse_buffers()
+  let g:test_is_flaky = 1
   CheckExecutable cat
 
   sp test_channel.vim
@@ -1884,6 +1890,7 @@
 endfunc
 
 func Test_cwd()
+  let g:test_is_flaky = 1
   let g:envstr = ''
   if has('win32')
     let expect = $TEMP