Start shared containers in postProcessTestInstance for PER_CLASS lifecycle

[remal]

When using @TestInstance(PER_CLASS) with Spring Boot, Quarkus, or similar frameworks, non-static @MethodSource factory methods may depend on static @Container fields being started. These factory methods run after postProcessTestInstance but before beforeAll, so the containers aren't started yet.

Example:

@TestInstance(TestInstance.Lifecycle.PER_CLASS)
@Testcontainers
@SpringBootTest
class UserServiceTest {

    @Container
    static PostgreSQLContainer<?> postgres = new PostgreSQLContainer<>("postgres:16");

    @Autowired
    UserService userService;

    // Non-static @MethodSource is possible with PER_CLASS.
    // Without the fix, postgres is not started yet when this runs.
    Stream<Arguments> userArguments() {
        return userService.findAllRoles(
                postgres.getJdbcUrl(), postgres.getUsername(), postgres.getPassword()
        ).stream().map(Arguments::of);
    }

    @ParameterizedTest
    @MethodSource("userArguments")
    void should_create_user_with_role(String role) {
        // ...
    }
}

With PER_CLASS, postProcessTestInstance is called exactly once per test class (same as beforeAll), so moving shared container startup there doesn't change the lifecycle guarantees. It just makes them available earlier, during @MethodSource resolution and for other TestInstancePostProcessor extensions.

I submitted a PR with an implementation: #11703

I should have opened this discussion first, apologies. Happy to hear your thoughts.