Expand specs for String#to_f #1925

Job	Run time
RuboCop	34s
specs (ubuntu, 3.1.6)	1m 33s
specs (ubuntu, 3.2.7)	1m 31s
specs (ubuntu, 3.3.7)	1m 26s
specs (ubuntu, 3.4.2)	1m 35s
specs (macos, 3.1.6)	1m 29s
specs (macos, 3.2.7)	1m 32s
specs (macos, 3.3.7)	1m 35s
specs (macos, 3.4.2)	1m 18s
specs (windows, 3.1.6)	4m 21s
specs (windows, 3.2.7)	3m 42s
specs (windows, 3.3.7)	3m 49s
specs (windows, 3.4.2)	4m 19s
specs (ubuntu, 3.4.2, --enable-frozen-string-literal)	1m 32s
	30m 16s

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