J2CL Java single-file unit testing

Tests.java

import elemental2.dom.DomGlobal;
import elemental2.promise.Promise;

import java.util.List;
import java.util.function.Supplier;

public class Tests {

    public static final String GREEN_CIRCLE = "\uD83D\uDFE2";
    public static final String RED_CIRCLE = "\uD83D\uDD34";

    public void runTests() {
        runTest("test1", this::test1);
        runTest("test2", this::test2);
        runTest("test3", this::test3);
    }

    public void test1() {
        String expected = "1";
        String actual = Integer.toString(1);
        assertEquals(expected, actual);
    }

    public void test2() {
        String expected = "2";
        String actual = Integer.toString(1);
        assertEquals(expected, actual);
    }

    private Promise<?> test3() {
        Promise<List<RatingItem>> promise = new ApiClient().fetchRatings();
        return promise.then(
            success -> {
                assertEquals(7412, success.size());
                return null;
            }
        );
    }

    private void runTest(String testName, Runnable testMethod) {
        String errorMsg;
        try {
            testMethod.run();
            errorMsg = null;
        } catch (Exception e) {
            errorMsg = e.toString();
        }
        DomGlobal.console.log("Test %s: %s", testName, errorMsg == null ? GREEN_CIRCLE : RED_CIRCLE + ". " + errorMsg);
    }

    private void runTest(String testName, Supplier<Promise<?>> testMethod) {
        try {
            Promise<?> promise = testMethod.get();
            promise.then(
                success -> {
                    DomGlobal.console.log("Test %s: %s", testName, GREEN_CIRCLE);
                    return null;
                },
                failure -> {
                    DomGlobal.console.log("Test %s: %s", testName, RED_CIRCLE + ". " + failure.toString());
                    return null;
                }
            );
        } catch (Exception e) {
            DomGlobal.console.log("Test %s: %s", testName, RED_CIRCLE + ". " + e);
        }
    }

    private void assertEquals(Object expected, Object actual) {
        if (!expected.equals(actual)) {
            throw new RuntimeException("Failed: expected " + expected + ", got " + actual);
        }
    }

}

// new Tests().runTests();