tests: oldtests: mark Test_cursorhold_insert as flaky [ci skip] (#10264)

Fails often on CI (OSX), e.g.:

```
1 FAILED:
Found errors in Test_cursorhold_insert():
function RunTheTest[37]..Test_cursorhold_insert line 9: Expected 1 but got 0
```

> Compiler: clang Xcode: xcode10.1 C

The test could be adjusted to re-try the timer a few times, but I do not
think it's really worth it currently, and that the test should be marked
as flaky instead.

src/nvim/testdir/runtest.vim

@@ -264,6 +264,7 @@ endif
 
 " Names of flaky tests.
 let s:flaky = [
+      \ 'Test_cursorhold_insert()',
       \ 'Test_exit_callback_interval()',
       \ 'Test_oneshot()',
       \ 'Test_out_cb()',