patch 8.0.1507: timer test is a bit flaky

Problem:    Timer test is a bit flaky.
Solution:   Add it to the list of flaky tests.

src/testdir/runtest.vim

@@ -258,6 +258,7 @@ let s:flaky = [
       \ 'Test_quoteplus()',
       \ 'Test_quotestar()',
       \ 'Test_reltime()',
+      \ 'Test_repeat_three()',
       \ 'Test_terminal_composing_unicode()',
       \ 'Test_terminal_noblock()',
       \ 'Test_terminal_redir_file()',