test: Skip meson integration tests if SYSTEMD_INTEGRATION_TESTS != 1

test: Skip meson integration tests if SYSTEMD_INTEGRATION_TESTS != 1

We cannot mark a test suite as excluded by default in meson. Instead,
let's require that SYSTEMD_INTEGRATION_TESTS=1 and skip any integration
test if it's not set. This is effectively the same as excluding it by
default. If the integration-test option is enabled, we'll set the
environment variable by default, just like we do with SYSTEMD_SLOW_TESTS
and the slow-tests meson option.