ConfineMetaClassChanges leaks metaClass change

Issue description

A clear and concise description.
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A metaClass change that should be confined with ConfineMetaClassChanges leaks and causes a test failure in other tests

How to reproduce:

• clone https://github.com/secretescapes/grails-test-bug
• ensure you're running Grails 2
• grails test-app unit: spock.bug.FooSpec spock.bug.BarSpec spock.bug.BazSpec

Link to a gist or similar (optional)

Commit with the relevant snippets:

https://github.com/secretescapes/grails-test-bug/commit/a65fa39f8db2179a14ca8e5b95b3e7ad1fe0fab7

Additional Environment information

Version of your build tool(if used), Java, Groovy, IDE, OS etc

Grails 2

Java/JDK

openjdk version "1.8.0_111"
OpenJDK Runtime Environment (build 1.8.0_111-8u111-b14-2ubuntu0.16.10.2-b14)
OpenJDK 64-Bit Server VM (build 25.111-b14, mixed mode)

Groovy version

Note that versions older than 2.0 are no longer supported.

2.4.6

Build tool version

Grails 2.5.5

Operating System

Linux

Grails 2 is unsupported, but this error should be impossible, whatever framework one is using. I hoped that the Grails developers would be able to shine some light upon the machinery that's causing ConfineMetaClassChanges to not work as expected, but maybe the sample project is enough for you to understand what's going wrong?

Clone the linked grails-test-bug project and run the provided Grails test-app command to reproduce the failure. Read the referenced commit and investigate ConfineMetaClassChanges around the failing specifications; done means the confined metaClass change no longer leaks and the listed tests pass.