test(plugins): add test suite for Plugins service

modules/Plugins/Core/BasePlugin.php

modules/Plugins/Core/Plugins.php

modules/Plugins/Helpers/plugins_helper.php

tests/modules/Plugins/ManifestTest.php

tests/modules/Plugins/PluginsTest.php

tests/modules/Plugins/mocks/manifests/manifest-empty.json

...odules/Plugins/mocks/manifests/manifest-full-invalid.json

.../modules/Plugins/mocks/manifests/manifest-full-valid.json

tests/modules/Plugins/mocks/manifests/manifest-required.json

...s/modules/Plugins/mocks/plugins/acme/all-hooks/Plugin.php

...odules/Plugins/mocks/plugins/acme/all-hooks/manifest.json

...ules/Plugins/mocks/plugins/acme/empty-manifest/Plugin.php

...s/Plugins/mocks/plugins/acme/empty-manifest/manifest.json

...ules/Plugins/mocks/plugins/acme/hello-universe/Plugin.php

...s/Plugins/mocks/plugins/acme/hello-universe/manifest.json

...modules/Plugins/mocks/plugins/acme/hello-world/Plugin.php

...ules/Plugins/mocks/plugins/acme/hello-world/manifest.json

...les/Plugins/mocks/plugins/acme/undeclared-hook/Plugin.php

.../Plugins/mocks/plugins/acme/undeclared-hook/manifest.json

tests/modules/Plugins/mocks/plugins/atlantis/empty/.gitkeep