vim-patch:8.2.0400: not all tests using a terminal are in the list of flaky tests

Problem:    Not all tests using a terminal are in the list of flaky tests.
Solution:   Introduce the test_is_flaky flag.
3cdcb090a6

Cherry-pick g:test_is_flaky from patch v8.2.0557.
Ensure that test_is_flaky variable is set from the global scope..

N/A patches for version.c:

vim-patch:8.2.2647: terminal test sometimes hangs

Problem:    Terminal test sometimes hangs.
Solution:   Wait for the shell to display a prompt.
f4a2ed0714

vim-patch:8.2.2648: terminal resize test sometimes hangs

Problem:    Terminal resize test sometimes hangs.
Solution:   Wait for the shell to display a prompt and other output.
c54f347d63

fixup! vim-patch:8.2.0400: not all tests using a terminal are in the list of flaky tests

src/nvim/testdir/runtest.vim

@@ -373,9 +373,6 @@ let s:flaky_tests = [
       \ 'Test_with_partial_callback()',
       \ ]
 
-" Pattern indicating a common flaky test failure.
-let s:flaky_errors_re = 'StopVimInTerminal\|VerifyScreenDump'
-
 " Locate Test_ functions and execute them.
 redir @q
 silent function /^Test_
@@ -410,6 +407,9 @@ for s:test in sort(s:tests)
   let total_errors = []
   let run_nr = 1
 
+  " A test can set g:test_is_flaky to retry running the test.
+  let g:test_is_flaky = 0
+
   call RunTheTest(s:test)
 
   " Repeat a flaky test.  Give up when:
@@ -417,7 +417,7 @@ for s:test in sort(s:tests)
   " - it fails five times (with a different message)
   if len(v:errors) > 0
         \ && (index(s:flaky_tests, s:test) >= 0
-        \      || v:errors[0] =~ s:flaky_errors_re)
+        \      || g:test_is_flaky)
     while 1
       call add(s:messages, 'Found errors in ' . s:test . ':')
       call extend(s:messages, v:errors)

src/nvim/testdir/test_autocmd.vim

@@ -1366,9 +1366,11 @@ func Test_TextChangedI_with_setline()
 endfunc
 
 func Test_Changed_FirstTime()
-  if !has('terminal') || has('gui_running')
-    return
-  endif
+  CheckFeature terminal
+  CheckNotGui
+  " Starting a terminal to run Vim is always considered flaky.
+  let test_is_flaky = 1
+
   " Prepare file for TextChanged event.
   call writefile([''], 'Xchanged.txt')
   let buf = term_start([GetVimProg(), '--clean', '-c', 'set noswapfile'], {'term_rows': 3})