Rewrite specs for String#to_c with special float values #1932

Job	Run time
specs (ubuntu, 3.1.6)	1m 31s
RuboCop	33s
specs (ubuntu, 3.2.7)	1m 30s
specs (ubuntu, 3.3.7)	1m 32s
specs (ubuntu, 3.4.2)	1m 36s
specs (macos, 3.1.6)	1m 33s
specs (macos, 3.2.7)	1m 18s
specs (macos, 3.3.7)	1m 17s
specs (macos, 3.4.2)	1m 26s
specs (windows, 3.1.6)	3m 43s
specs (windows, 3.2.7)	4m 8s
specs (windows, 3.3.7)	3m 33s
specs (windows, 3.4.2)	4m 48s
specs (ubuntu, 3.4.2, --enable-frozen-string-literal)	1m 34s
	30m 2s

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