Fix test expectation for single digit days

See <https://github.com/php/php-src/pull/16747#pullrequestreview-2444293792>.

Co-authored-by: Ayesh Karunaratne <ayesh@aye.sh>

Add PHP_BUILD_DATE constant

This information can be occasionally useful, and would otherwise need
to be parsed from `phpinfo()` output.

However, maybe more importantly we unify

Add PHP_BUILD_DATE constant

This information can be occasionally useful, and would otherwise need
to be parsed from `phpinfo()` output.

However, maybe more importantly we unify the build date between what is
given by `php -v` and `php -i`, since these compilation units are not
necessarily preprocessed within the same second.

Closes GH-16747.

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