Add a spec for `autoload` with `private_constant` #2071

Job	Run time
RuboCop	50s
specs (ubuntu, 3.4.4)	1m 43s
specs (macos, 3.3.8)	1m 35s
specs (macos, 3.4.4)	2m 9s
specs (macos, 3.2.8)	1m 39s
specs (windows, 3.3.8)	3m 10s
specs (ubuntu, 3.3.8)	1m 30s
specs (windows, 3.2.8)	3m 23s
specs (ubuntu, 3.2.8)	1m 31s
specs (windows, 3.4.4)	3m 3s
specs (ubuntu, 3.4.4, --enable-frozen-string-literal)	1m 37s
specs (ubuntu, 3.4.4, --parser=parse.y)	1m 38s
	23m 48s

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