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.

src/test/meson.build

@@ -15,6 +15,7 @@ test_env.set('SYSTEMD_LANGUAGE_FALLBACK_MAP', language_fallback_map)
 test_env.set('PATH', project_build_root + ':' + path)
 test_env.set('PROJECT_BUILD_ROOT', project_build_root)
 test_env.set('SYSTEMD_SLOW_TESTS', slow_tests ? '1' : '0')
+test_env.set('SYSTEMD_INTEGRATION_TESTS', integration_tests ? '1' : '0')
 
 if efi_addon != ''
         test_env.set('EFI_ADDON', efi_addon)