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.

[CLAassistant]

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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.}

[jdaugherty]

@jchharris while the upstream bug fix appears to have been merged, I can't find evidence this was ever merged into the test suite. Would you mind signing the CLA so I can get this change merged?

[jdaugherty]

@jchharris If you can get the CLA signed, I can contribute this change into grails-core (this repo has moved)