Fix failsGracefullyWhenNoTestMainMethodIsFound on Windows

spring-boot-project/spring-boot-tools/spring-boot-gradle-plugin/src/test/java/org/springframework/boot/gradle/tasks/run/BootTestRunIntegrationTests.java

@@ -113,7 +113,7 @@ class BootTestRunIntegrationTests {
 		else {
 			assertThat(result.getOutput())
 				.contains("Main class name has not been configured and it could not be resolved from classpath "
-						+ "build/classes/java/test");
+						+ String.join(File.separator, "build", "classes", "java", "test"));
 		}
 	}