Only load PHPUnit dependencies if using PHPUnit

[sirbrillig]

It might be best to move the PHPUnit assertions to a separate library or at least conditionally load it only if we are inside PHPUnit. That would allow Spies to be more easily re-used in other test runners such as corretto or atoum.

See also #21, which is related.

• Separate PHPUnit from Expectations
• Conditionally load PHPUnit assertions only if PHPUnit is available
• Move phpunit (and patchwork) from dependency to dev dependency