Build/Test Tools: Fix installation tests for PHP <= 7.3.

In MySQL 8.0, the default authentication plugin changed from `mysql_native_password` to `caching_sha2_password`, which is not available in PHP 7.3 and earlier. The installation testing workflow currently accounts for this, but only for PHP 7.2 & 7.3.

It also does not account for a change in MySQL 8.4 which changed the related flag for controlling this from `--default-authentication-plugin` to `--authentication-policy`.

This adjusts the flags and surrounding conditions to ensure the correct ones are passed for the right combinations.

Props johnbillion.
See #64225, #64227.

git-svn-id: https://develop.svn.wordpress.org/trunk@62022 602fd350-edb4-49c9-b593-d223f7449a82