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.

According to commit 0eea9a6[1], these tests fail with old Oniguruma
versions; we are not sure which version of Oniguruma is required to let
them pass, but at least 6.9.3 is sufficient.

[1] <http://git.php.net/?p=php-src.git;a=commit;h=0eea9a642941ab5d4c612f8092f186977afbb73e>

* PHP-7.3:
  Fix #78609: mb_check_encoding() no longer supports stringable objects

* PHP-7.2:
  Fix #78609: mb_check_encoding() no longer supports stringable objects

We apply type juggling for other types than array.

* PHP-7.3:
  Fix #78559: Heap buffer overflow in mb_eregi

We backport kkos/oniguruma@d3e402928b6eb3327f8f7d59a9edfa622fec557b.

* PHP-7.3:
  Fix #78579: mb_decode_numericentity: args number inconsistency

* PHP-7.2:
  Fix #78579: mb_decode_numericentity: args number inconsistency