This test is somewhat fragile in that it depends on how well a
particular regex is optimized. Apparently on 6.9.1 this regex
would hit the default retry_limit of 1000000 already. I'm limiting
this to 6.9.3 because that's the version that works for me.
This is very similar to the existing mbstring.regex_stack_limit,
but for backtracking. The default value matches pcre.backtrack_limit.
Only used on libonig >= 2.8.0.