Remove empty constructors and suite() methods in MigrateJUnitTestCase (#926)

After removing `super(testName)`, constructors left with an empty body
are now fully removed. Static `suite()` methods returning
`junit.framework.Test` or `TestSuite` are also removed along with their
stale imports.

Fixes moderneinc/customer-requests#1948

Author: timtebeek

src/main/java/org/openrewrite/java/testing/junit5/MigrateJUnitTestCase.java

@@ -119,9 +119,28 @@ public J.ClassDeclaration visitClassDeclaration(J.ClassDeclaration classDecl, Ex
         }
 
         @Override
-        public J.MethodDeclaration visitMethodDeclaration(J.MethodDeclaration method, ExecutionContext ctx) {
+        public J.@Nullable MethodDeclaration visitMethodDeclaration(J.MethodDeclaration method, ExecutionContext ctx) {
             J.MethodDeclaration md = super.visitMethodDeclaration(method, ctx);
             updateCursor(md);
+
+            // After super.visitMethodDeclaration (which triggers visitMethodInvocation
+            // to remove super(testName)), check if this is now an empty constructor
+            if (md.isConstructor() &&
+                (md.getBody() == null || md.getBody().getStatements().isEmpty())) {
+                return null;
+            }
+
+            // Remove suite() methods that return junit.framework.Test or TestSuite
+            if ("suite".equals(md.getSimpleName()) &&
+                md.hasModifier(J.Modifier.Type.Static) &&
+                md.getMethodType() != null &&
+                (TypeUtils.isOfClassType(md.getMethodType().getReturnType(), "junit.framework.Test") ||
+                 TypeUtils.isOfClassType(md.getMethodType().getReturnType(), "junit.framework.TestSuite"))) {
+                maybeRemoveImport("junit.framework.Test");
+                maybeRemoveImport("junit.framework.TestSuite");
+                return null;
+            }
+
             if (md.getSimpleName().startsWith("test") && md.getLeadingAnnotations().stream().noneMatch(JUNIT_TEST_ANNOTATION_MATCHER::matches)) {
                 md = updateMethodDeclarationAnnotationAndModifier(md, "@Test", "org.junit.jupiter.api.Test", ctx);
             } else if ("setUp".equals(md.getSimpleName()) && md.getLeadingAnnotations().stream().noneMatch(JUNIT_BEFORE_ANNOTATION_MATCHER::matches)) {

src/test/java/org/openrewrite/java/testing/junit5/MigrateJUnitTestCaseTest.java

@@ -354,7 +354,72 @@ public AppTest(String testName) {
               """,
             """
               public class AppTest {
+              }
+              """
+          )
+        );
+    }
+
+    @Test
+    void constructorWithAdditionalStatementsIsKept() {
+        //language=java
+        rewriteRun(
+          java(
+            """
+              import junit.framework.TestCase;
+
+              public class AppTest extends TestCase {
+                  private final String name;
                   public AppTest(String testName) {
+                      super(testName);
+                      this.name = testName;
+                  }
+              }
+              """,
+            """
+              public class AppTest {
+                  private final String name;
+                  public AppTest(String testName) {
+                      this.name = testName;
+                  }
+              }
+              """
+          )
+        );
+    }
+
+    @Test
+    void suiteMethodIsRemoved() {
+        //language=java
+        rewriteRun(
+          java(
+            """
+              import junit.framework.Test;
+              import junit.framework.TestCase;
+              import junit.framework.TestSuite;
+
+              public class AppTest extends TestCase {
+                  public AppTest(String testName) {
+                      super(testName);
+                  }
+                  public static Test suite() {
+                      return new TestSuite(AppTest.class);
+                  }
+                  public void testApp() {
+                      assertTrue(true);
+                  }
+              }
+              """,
+            """
+              import org.junit.jupiter.api.Test;
+
+              import static org.junit.jupiter.api.Assertions.assertTrue;
+
+              public class AppTest {
+
+                  @Test
+                  public void testApp() {
+                      assertTrue(true);
                   }
               }
               """