Add a spec for `autoload` with `private_constant` #2070

Job	Run time
RuboCop	52s
specs (ubuntu, 3.2.8)	1m 29s
specs (ubuntu, 3.3.8)	1m 27s
specs (macos, 3.3.8)	1m 36s
specs (ubuntu, 3.4.4)	1m 43s
specs (macos, 3.2.8)	1m 43s
specs (windows, 3.3.8)	3m 18s
specs (macos, 3.4.4)	1m 15s
specs (ubuntu, 3.4.4, --enable-frozen-string-literal)	1m 39s
specs (windows, 3.2.8)	3m 12s
specs (ubuntu, 3.4.4, --parser=parse.y)	1m 43s
specs (windows, 3.4.4)	2m 58s
	22m 55s

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