Cleanup: Provide a release test ProperMethodCategorizationTest>>#testDisplayStringMethodNeedsToBeInDisplayingProtocol

[astares]

Implement a ProperMethodCategorizationTest>>#testDisplayStringMethodNeedsToBeInDisplayingProtocol

testDisplayStringMethodNeedsToBeInDisplayingProtocol
	"The #displayString method should be in method protocol 'displaying'"

	self assureAll: #displayString areCategorizedIn: #displaying whenSubclassOf: Object

Requires https://github.com/pharo-project/pharo/issues/16050 to be fixed first and other cases:

to become green