Adding test cases for grails/grails-core#11056

[jchharris]

Functional Test case added which performs the following:

• Load a child application context
• Check that the grailsApplication reference is still set
• Unload a child application context
• Check that the grailsApplication is still set and is the same reference as before the child context was loaded.

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[CLAassistant]

Thank you for your submission! We really appreciate it. Like many open source projects, we ask that you sign our Contributor License Agreement before we can accept your contribution.
_{You have signed the CLA already but the status is still pending? Let us recheck it.}