Add spec for `defined?` on constant with const_missing #1806

Job	Run time
RuboCop	30s
specs (ubuntu, 3.0.6)	1m 27s
specs (ubuntu, 3.1.4)	1m 27s
specs (ubuntu, 3.2.2)	1m 26s
specs (ubuntu, 3.3.0)	1m 27s
specs (macos, 3.0.6)	1m 26s
specs (macos, 3.1.4)	1m 29s
specs (macos, 3.2.2)	1m 11s
specs (macos, 3.3.0)	1m 11s
specs (windows, 3.0.6)	4m 12s
specs (windows, 3.1.4)	4m 51s
specs (windows, 3.2.2)	4m 52s
specs (windows, 3.3.0)	4m 58s
specs (ubuntu, 3.3.0, --enable-frozen-string-literal)	1m 22s
	31m 49s

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