chore: add support for PHPUnit 8

[bshaffer]

Adds PHPUnit 8 for testing!!
This will allow us to test on PHP 8.0 and 8.1.

Some important notes:

• PHPUnit 9 was not used because it threw an unbelievable amount of warnings for phpunit-prophecy, and there is no polyfill available for that yet.
• I used Yoast PHPUnit Polyfill to all composer.json to keep compatibility with PHPUnit 4 and 5.
  • PHPUnit 4 is needed for testing on PHP 5.5
  • PHPUnit 5 is needed for testing on PHP 5.6, 7.0, and 7.1
• I removed the custom ResultPrinter class we were using for snippets test. This was causing function signature conflicts between PHPUnit versions, and it didn't seem worth the headache to make it compatible (which is non-trivial).
• Once we drop support for PHP 7.3 and below, we can upgrade to PHPUnit 9.0 and remove all the polyfill logic. We will not be able to upgrade to PHPUnit 10, even if it's available by then, because it requires PHP 8.1+
• Once this is merged, I will add tests for PHP 8.0 and 8.1 in a separate PR.

[bshaffer]

These additional lines make sure that the tests run the emulator using the current branch for the google/cloud-core dependency, instead of using a tagged version. This is required for this PR because we made changes in Core that these tests need to pass.