Add TemporaryClasspathExecutor test utility (#4544)

This commit extracts the logic previously in `LauncherFactoryTests` to execute code with an extra classpath root, which is required for SPI-based tests.

Author: scordio

junit-jupiter-api/src/testFixtures/java/org/junit/jupiter/api/TemporaryClasspathExecutor.java

@@ -0,0 +1,54 @@
+/*
+ * Copyright 2015-2025 the original author or authors.
+ *
+ * All rights reserved. This program and the accompanying materials are
+ * made available under the terms of the Eclipse Public License v2.0 which
+ * accompanies this distribution and is available at
+ *
+ * https://www.eclipse.org/legal/epl-v20.html
+ */
+
+package org.junit.jupiter.api;
+
+import java.io.IOException;
+import java.io.UncheckedIOException;
+import java.net.URL;
+import java.net.URLClassLoader;
+
+/**
+ * Utility class for executing code with a temporary classpath.
+ *
+ * @since 5.13
+ */
+public class TemporaryClasspathExecutor {
+
+	private TemporaryClasspathExecutor() {
+	}
+
+	/**
+	 * Execute the {@link Runnable} within a custom classpath, temporarily modifying the
+	 * thread's {@link Thread#getContextClassLoader() context class loader} to include
+	 * the provided {@code classpathRoot}.
+	 *
+	 * <p>After the given {@code Runnable} completes, the original context class loader is
+	 * restored.
+	 *
+	 * @param classpathRoot the root path to be added to the classpath, resolved relative
+	 * to the current thread's context class loader.
+	 * @param runnable the {@code Runnable} to execute with the temporary classpath.
+	 */
+	public static void withAdditionalClasspathRoot(String classpathRoot, Runnable runnable) {
+		var current = Thread.currentThread().getContextClassLoader();
+		try (var classLoader = new URLClassLoader(new URL[] { current.getResource(classpathRoot) }, current)) {
+			Thread.currentThread().setContextClassLoader(classLoader);
+			runnable.run();
+		}
+		catch (IOException e) {
+			throw new UncheckedIOException(e);
+		}
+		finally {
+			Thread.currentThread().setContextClassLoader(current);
+		}
+	}
+
+}

platform-tests/src/test/java/org/junit/platform/launcher/core/LauncherFactoryTests.java

@@ -14,15 +14,12 @@
 import static org.junit.jupiter.api.Assertions.assertFalse;
 import static org.junit.jupiter.api.Assertions.assertThrows;
 import static org.junit.jupiter.api.Assertions.assertTrue;
+import static org.junit.jupiter.api.TemporaryClasspathExecutor.withAdditionalClasspathRoot;
 import static org.junit.platform.engine.discovery.DiscoverySelectors.selectClass;
 import static org.junit.platform.launcher.LauncherConstants.DEACTIVATE_LISTENERS_PATTERN_PROPERTY_NAME;
 import static org.junit.platform.launcher.LauncherConstants.ENABLE_LAUNCHER_INTERCEPTORS;
 import static org.junit.platform.launcher.core.LauncherDiscoveryRequestBuilder.request;
 
-import java.io.IOException;
-import java.io.UncheckedIOException;
-import java.net.URL;
-import java.net.URLClassLoader;
 import java.util.concurrent.atomic.AtomicReference;
 import java.util.logging.LogRecord;
 
@@ -434,18 +431,7 @@ private static void withSystemProperty(String key, String value, Runnable runnab
 	}
 
 	private static void withTestServices(Runnable runnable) {
-		var current = Thread.currentThread().getContextClassLoader();
-		var url = LauncherFactoryTests.class.getClassLoader().getResource("testservices/");
-		try (var classLoader = new URLClassLoader(new URL[] { url }, current)) {
-			Thread.currentThread().setContextClassLoader(classLoader);
-			runnable.run();
-		}
-		catch (IOException e) {
-			throw new UncheckedIOException(e);
-		}
-		finally {
-			Thread.currentThread().setContextClassLoader(current);
-		}
+		withAdditionalClasspathRoot("testservices/", runnable);
 	}
 
 	private LauncherDiscoveryRequest createLauncherDiscoveryRequestForBothStandardEngineExampleClasses() {