Generate code coverage report

Summary:
make coverage allows to create a code coverage report directly with PHPUnit.

Reuse the .phpunit.cache/ directory, as already set in .gitignore,
to avoid clutter the build.

Ref T1637 for use in Jenkins pipeline.

Test Plan: make coverage

Maniphest Tasks: T1637

Differential Revision: https://devcentral.nasqueron.org/D3839