Polish "Tighten up PropertiesLauncher's contract"

See gh-8346
Closes gh-7221

spring-boot-tools/spring-boot-loader/src/test/java/org/springframework/boot/loader/PropertiesLauncherTests.java

@@ -1,5 +1,5 @@
 /*
- * Copyright 2012-2016 the original author or authors.
+ * Copyright 2012-2017 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.

spring-boot-tools/spring-boot-loader/src/test/resources/BOOT-INF/classes/application.properties

@@ -1 +1 @@
-loader.main: demo.NoSuchApplication
+loader.main: demo.Application

spring-boot-tools/spring-boot-loader/src/test/resources/placeholders/META-INF/MANIFEST.MF

@@ -0,0 +1,2 @@
+Manifest-Version: 1.0
+Start-Class: ${foo.main}