Enable expose_php in the PHP-FPM test fixture

[mnapoli]

Problem

Three FpmHandlerTest cases fail on a stock Debian/Ubuntu PHP install:

1) Bref\Test\FpmRuntime\FpmHandlerTest::test response with headers
Failed asserting that '' starts with "PHP/".
2) Bref\Test\FpmRuntime\FpmHandlerTest::test response with multivalue headers
3) Bref\Test\FpmRuntime\FpmHandlerTest::test response with error_log

They assert on the X-Powered-By header, which PHP only sends when expose_php is on. The Bref
layers enable it, so the assertion is right about production — but the distribution packages ship
expose_php = Off in /etc/php/<version>/fpm/php.ini (while leaving it on for the CLI SAPI), so
the tests depend on how the host PHP happens to be configured.

Found while setting up a Cloud Agent environment where PHP comes from ppa:ondrej/php.