Add test for String#to_r with leading plus sign #381

Job	Run time
RuboCop	30s
specs (ubuntu, 3.1.6)	1m 32s
specs (ubuntu, 3.2.6)	1m 30s
specs (ubuntu, 3.3.7)	1m 25s
specs (ubuntu, 3.4.1)	1m 39s
specs (macos, 3.1.6)	1m 25s
specs (macos, 3.2.6)	1m 15s
specs (macos, 3.3.7)	1m 17s
specs (macos, 3.4.1)	1m 39s
specs (windows, 3.1.6)	3m 43s
specs (windows, 3.2.6)	3m 54s
specs (windows, 3.3.7)	3m 33s
specs (windows, 3.4.1)	3m 48s
specs (ubuntu, 3.3.0, --enable-frozen-string-literal)	1m 25s
	28m 35s

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