Add spec for Warning.categories #2014

Job	Run time
RuboCop	56s
specs (ubuntu, 3.2.7)	1m 26s
specs (ubuntu, 3.3.7)	1m 27s
specs (ubuntu, 3.4.2)	1m 34s
specs (macos, 3.2.7)	1m 27s
specs (macos, 3.3.7)	1m 11s
specs (macos, 3.4.2)	1m 19s
specs (windows, 3.2.7)	3m 4s
specs (windows, 3.3.7)	3m 7s
specs (windows, 3.4.2)	3m 3s
specs (ubuntu, 3.4.2, --enable-frozen-string-literal)	1m 35s
specs (ubuntu, 3.4.2, --parser=parse.y)	1m 41s
	21m 50s

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