Carefully add nested archives from JAR in PropertiesLauncher

If user runs an executable archive then it and its lib directory will be
on the classpath. Entries from loader.path take precedence in a way that
should make sense to users (earlier wins like in CLASSPATH env var).

Also added new integration tests to verify the behaviour (big improvement
on the old ones, which probably aought to be beefed up to the same
standard).

Fixes gh-2314

spring-boot-tools/spring-boot-loader/src/it/executable-props/verify.groovy

@@ -0,0 +1,32 @@
+def jarfile = './target/executable-props-0.0.1.BUILD-SNAPSHOT-full.jar'
+
+new File("${basedir}/application.properties").delete()
+
+String exec(String command) {
+	def proc = command.execute([], basedir)
+	proc.waitFor()
+	proc.err.text
+}
+
+String out = exec("java -jar ${jarfile}")
+assert out.contains('Hello Embedded World!'),
+	'Using -jar my.jar should use the application.properties from the jar\n' + out
+
+out = exec("java -cp ${jarfile} org.springframework.boot.loader.PropertiesLauncher")
+assert out.contains('Hello Embedded World!'),
+	'Using -cp my.jar with PropertiesLauncher should use the application.properties from the jar\n' + out
+
+new File("${basedir}/application.properties").withWriter { it -> it << "message: Foo" }
+out = exec("java -jar ${jarfile}")
+assert out.contains('Hello Embedded World!'),
+	'Should use the application.properties from the jar in preference to local filesystem\n' + out
+
+out = exec("java -Dloader.path=.,lib -jar ${jarfile}")
+assert out.contains('Hello Embedded Foo!'),
+	'With loader.path=.,lib should use the application.properties from the local filesystem\n' + out
+
+new File("${basedir}/target/application.properties").withWriter { it -> it << "message: Spam" }
+
+out = exec("java -Dloader.path=target,.,lib -jar ${jarfile}")
+assert out.contains('Hello Embedded Spam!'),
+	'With loader.path=target,.,lib should use the application.properties from the target directory\n' + out