Add C extension spec for rb_undefine_finalizer #369

Job	Run time
RuboCop	30s
specs (ubuntu, 3.0.6)	1m 32s
specs (ubuntu, 3.1.4)	1m 32s
specs (ubuntu, 3.2.2)	1m 32s
specs (ubuntu, 3.3.0)	1m 32s
specs (ubuntu, 3.4.1)	1m 29s
specs (macos, 3.0.6)	1m 34s
specs (macos, 3.1.4)	1m 26s
specs (macos, 3.2.2)	1m 22s
specs (macos, 3.3.0)	1m 16s
specs (macos, 3.4.1)	1m 16s
specs (windows, 3.0.6)	3m 39s
specs (windows, 3.1.4)	5m 29s
specs (windows, 3.2.2)	4m 53s
specs (windows, 3.3.0)	4m 30s
specs (windows, 3.4.1)	3m 55s
specs (ubuntu, 3.3.0, --enable-frozen-string-literal)	1m 26s
	38m 53s

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