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.
diff --git a/src/testdir/runtest.vim b/src/testdir/runtest.vim
index d8a1cf7..b7f9229 100644
--- a/src/testdir/runtest.vim
+++ b/src/testdir/runtest.vim
@@ -379,9 +379,6 @@
       \ 'Test_zz1_terminal_in_gui()',
       \ ]
 
-" 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 @@
   let total_errors = []
   let run_nr = 1
 
+  " A test can set test_is_flaky to retry running the test.
+  let test_is_flaky = 0
+
   call RunTheTest(s:test)
 
   " Repeat a flaky test.  Give up when:
@@ -417,7 +417,7 @@
   " - 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)
+        \      || test_is_flaky)
     while 1
       call add(s:messages, 'Found errors in ' . s:test . ':')
       call extend(s:messages, v:errors)