Add specs for Refinement#target #1816

Job	Run time
RuboCop	30s
specs (ubuntu, 3.0.6)	1m 28s
specs (ubuntu, 3.1.4)	1m 28s
specs (ubuntu, 3.2.2)	1m 27s
specs (ubuntu, 3.3.0)	1m 23s
specs (macos, 3.0.6)	1m 29s
specs (macos, 3.1.4)	1m 21s
specs (macos, 3.2.2)	1m 8s
specs (macos, 3.3.0)	1m 21s
specs (windows, 3.0.6)	4m 59s
specs (windows, 3.1.4)	4m 18s
specs (windows, 3.2.2)	5m 12s
specs (windows, 3.3.0)	5m 23s
specs (ubuntu, 3.3.0, --enable-frozen-string-literal)	1m 22s
	32m 49s

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