patch 8.0.1262: terminal redir test is flaky

Problem:    Terminal redir test is flaky.
Solution:   Add it to the list of flaky tests.

src/testdir/runtest.vim

@@ -252,6 +252,7 @@ let s:flaky = [
       \ 'Test_reltime()',
       \ 'Test_terminal_composing_unicode()',
       \ 'Test_terminal_noblock()',
+      \ 'Test_terminal_redir_file()',
       \ 'Test_with_partial_callback()',
       \ ]