patch 8.0.1262: terminal redir test is flaky

Problem:    Terminal redir test is flaky.
Solution:   Add it to the list of flaky tests.
diff --git a/src/testdir/runtest.vim b/src/testdir/runtest.vim
index a359448..08c0e68 100644
--- a/src/testdir/runtest.vim
+++ b/src/testdir/runtest.vim
@@ -252,6 +252,7 @@
       \ 'Test_reltime()',
       \ 'Test_terminal_composing_unicode()',
       \ 'Test_terminal_noblock()',
+      \ 'Test_terminal_redir_file()',
       \ 'Test_with_partial_callback()',
       \ ]