Introduce BehaviorAssertion to implement BDD-style assertions

tests-core/src/main/java/com/pragmaticobjects/oo/tests/bdd/BehaviorAssertion.java

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+package com.pragmaticobjects.oo.tests.bdd;
+
+import com.pragmaticobjects.oo.tests.Assertion;
+
+public final class BehaviorAssertion<S, R> implements Assertion {
+
+    private final Step.Given<S> given;
+    private final Step.When<S, R> when;
+    private final Step.Then<R> then;
+
+    public BehaviorAssertion(
+            final Step.Given<S> given,
+            final Step.When<S, R> when,
+            final Step.Then<R> then
+    ) {
+        this.given = given;
+        this.when = when;
+        this.then = then;
+    }
+
+    @Override
+    public void check() throws Exception {
+        then.check(
+                when.when(
+                        given.given()
+                )
+        );
+    }
+}

tests-core/src/main/java/com/pragmaticobjects/oo/tests/bdd/Step.java

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+package com.pragmaticobjects.oo.tests.bdd;
+
+public interface Step {
+    interface Given<S> {
+        S given();
+    }
+
+    interface When<S, R> {
+        R when(final S sut);
+    }
+
+    interface Then<R> {
+        void check(final R result);
+    }
+}

tests-core/src/test/java/com/pragmaticobjects/oo/tests/bdd/BehaviorAssertionTest.java

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+package com.pragmaticobjects.oo.tests.bdd;
+
+import org.junit.jupiter.api.Test;
+
+import static org.assertj.core.api.Assertions.assertThatCode;
+import static org.assertj.core.api.Assertions.assertThatThrownBy;
+
+public class BehaviorAssertionTest {
+    @Test
+    public void assertAssertionFailsTest() {
+        assertThatThrownBy(
+                () ->
+                        new BehaviorAssertion<>(
+                                new GivenNop(),
+                                new WhenNop(),
+                                new ThenFails()
+                        ).check()
+        ).isInstanceOf(AssertionError.class);
+    }
+
+    @Test
+    public void assertAssertionPassesTest() {
+        assertThatCode(
+                () ->
+                        new BehaviorAssertion<>(
+                                new GivenNop(),
+                                new WhenNop(),
+                                new ThenSucceeds()
+                        ).check()
+        ).doesNotThrowAnyException();
+    }
+}

tests-core/src/test/java/com/pragmaticobjects/oo/tests/bdd/GivenNop.java

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+package com.pragmaticobjects.oo.tests.bdd;
+
+public final class GivenNop implements Step.Given<Void> {
+    @Override
+    public Void given() {
+        return null;
+    }
+}

tests-core/src/test/java/com/pragmaticobjects/oo/tests/bdd/ThenFails.java

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+package com.pragmaticobjects.oo.tests.bdd;
+
+import static org.assertj.core.api.Assertions.fail;
+
+public final class ThenFails implements Step.Then<Void> {
+    @Override
+    public void check(final Void result) {
+        fail("FAIL, just as planned");
+    }
+}

tests-core/src/test/java/com/pragmaticobjects/oo/tests/bdd/ThenSucceeds.java

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+package com.pragmaticobjects.oo.tests.bdd;
+
+public final class ThenSucceeds implements Step.Then<Void> {
+    @Override
+    public void check(final Void result) {
+    }
+}

tests-core/src/test/java/com/pragmaticobjects/oo/tests/bdd/WhenNop.java

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+package com.pragmaticobjects.oo.tests.bdd;
+
+public final class WhenNop implements Step.When<Void, Void> {
+    @Override
+    public Void when(final Void sut) {
+        return null;
+    }
+}