Add JUnit 5 ModifiedClassPathExtension

Add a JUnit 5 extension that allows tests to be run with a
modified classpath. Since JUnit 5 does not currently offer a way
to run tests with a different classpath, we instead fake the
original invocation and launch an entirely new run for each
method.

See gh-17491

spring-boot-project/spring-boot-tools/spring-boot-test-support/pom.xml

@@ -70,6 +70,10 @@
 			<groupId>org.junit.jupiter</groupId>
 			<artifactId>junit-jupiter</artifactId>
 		</dependency>
+		<dependency>
+			<groupId>org.junit.platform</groupId>
+			<artifactId>junit-platform-launcher</artifactId>
+		</dependency>
 		<dependency>
 			<groupId>org.mockito</groupId>
 			<artifactId>mockito-core</artifactId>