Test that auto configurations can be added by init scripts

When an init command is run, it may add entries to the classpath. This
commit adds a test that verifies that, if an entry that is added to
the classpath contains a CompilerAutoConfiguration file in
META-INF/services, then the CompilerAutoConfigurations declared in it
are found by subsequent ServiceLoader.load calls.

spring-boot-cli/src/test/plugins/custom/META-INF/services/org.springframework.boot.cli.compiler.CompilerAutoConfiguration

@@ -0,0 +1 @@
+cli.command.CustomCompilerAutoConfiguration