Introduce TestSocketUtils as a replacement for SocketUtils

SocketUtils was officially deprecated in 5.3.16 (gh-28052) and removed
in 6.0 M3 (gh-28054); however, there is still need for a subset of this
functionality in integration tests for testing scenarios in which it is
not possible for the system under test to select its own random port
(or rely on the operating system to provide an ephemeral port).

This commit therefore introduces a scaled down version in the
spring-test module called TestSocketUtils which supports retrieval of a
single TCP port.

See gh-29132

spring-test/src/test/java/org/springframework/test/util/TestSocketUtilsTests.java

@@ -0,0 +1,55 @@
+/*
+ * Copyright 2002-2022 the original author or authors.
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *      https://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+
+package org.springframework.test.util;
+
+import javax.net.ServerSocketFactory;
+
+import org.junit.jupiter.api.Test;
+import org.mockito.MockedStatic;
+import org.mockito.Mockito;
+
+import static org.assertj.core.api.Assertions.assertThat;
+import static org.assertj.core.api.Assertions.assertThatIllegalStateException;
+
+/**
+ * Unit tests for {@link  TestSocketUtils}.
+ *
+ * @author Sam Brannen
+ * @author Gary Russell
+ * @author Chris Bono
+ */
+class TestSocketUtilsTests {
+
+	@Test
+	void findAvailableTcpPort() {
+		int port = TestSocketUtils.findAvailableTcpPort();
+		assertThat(port >= 1024).as("port [" + port + "] >= " + 1024).isTrue();
+		assertThat(port <= 65535).as("port [" + port + "] <= " + 65535).isTrue();
+	}
+
+	@Test
+	void findAvailableTcpPortWhenNoAvailablePortFoundInMaxAttempts()  {
+		try (MockedStatic<ServerSocketFactory> mockedServerSocketFactory = Mockito.mockStatic(ServerSocketFactory.class)) {
+			mockedServerSocketFactory.when(ServerSocketFactory::getDefault).thenThrow(new RuntimeException("Boom"));
+			assertThatIllegalStateException().isThrownBy(TestSocketUtils::findAvailableTcpPort)
+					.withMessageStartingWith("Could not find an available TCP port")
+					.withMessageEndingWith("after 1000 attempts");
+
+		}
+	}
+
+}