Add C extension spec for rb_undefine_finalizer #1863

Job	Run time
RuboCop	29s
specs (ubuntu, 3.0.6)	1m 32s
specs (ubuntu, 3.1.4)	1m 32s
specs (ubuntu, 3.2.2)	1m 29s
specs (ubuntu, 3.3.0)	1m 27s
specs (macos, 3.0.6)	1m 39s
specs (macos, 3.1.4)	1m 24s
specs (macos, 3.2.2)	1m 16s
specs (macos, 3.3.0)	1m 13s
specs (windows, 3.0.6)	3m 53s
specs (windows, 3.1.4)	4m 14s
specs (windows, 3.2.2)	3m 55s
specs (windows, 3.3.0)	5m 4s
specs (ubuntu, 3.3.0, --enable-frozen-string-literal)	1m 24s
	30m 31s

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