Add specs for rb_interned_str and rb_interned_str_cstr #2275

Job	Run time
RuboCop	48s
specs (macos, 3.4.8)	1m 17s
specs (ubuntu, 4.0.0)	1m 34s
specs (ubuntu, 4.0.0, --enable-frozen-string-literal)	1m 28s
specs (windows, 3.4.8)	3m 19s
specs (ubuntu, 3.2.9)	1m 32s
specs (ubuntu, 3.3.10)	1m 32s
specs (windows, 3.3.10)	3m 15s
specs (macos, 4.0.0)	1m 23s
specs (windows, 4.0.0)	3m 38s
specs (macos, 3.2.9)	1m 26s
specs (ubuntu, 3.4.8)	1m 39s
specs (ubuntu, 4.0.0, --repeat 2)	2m 13s
specs (windows, 3.2.8)	3m 27s
specs (ubuntu, 4.0.0, --parser=parse.y)	1m 31s
specs (macos, 3.3.10)	2m 17s
	32m 19s

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