Add `expect_syntax_error` spec helper #1727

Job	Run time
RuboCop	24s
specs (ubuntu, 3.0.6)	1m 33s
specs (ubuntu, 3.1.4)	1m 35s
specs (ubuntu, 3.2.2)	1m 26s
specs (ubuntu, 3.3.0)	1m 29s
specs (macos, 3.0.6)	2m 20s
specs (macos, 3.1.4)	1m 37s
specs (macos, 3.2.2)	1m 14s
specs (macos, 3.3.0)	1m 12s
specs (windows, 3.0.6)	3m 21s
specs (windows, 3.1.4)	4m 1s
specs (windows, 3.2.2)	5m 26s
specs (windows, 3.3.0)	5m 5s
specs (ubuntu, 3.3.0, --enable-frozen-string-literal)	1m 28s
	32m 11s

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