Impove tests for error messages in Array#pack, String#unpack #2202

Job	Run time
RuboCop	59s
specs (macos, 3.3.9)	1m 36s
specs (ubuntu, 3.4.7)	1m 36s
specs (macos, 3.2.9)	50s
specs (windows, 3.4.7)	3m 33s
specs (ubuntu, 3.3.9)	1m 28s
specs (ubuntu, 3.4.7, --repeat 2)	2m 28s
specs (ubuntu, 3.2.9)	1m 2s
specs (windows, 3.3.9)	3m 25s
specs (windows, 3.2.8)	3m 39s
specs (macos, 3.4.7)	1m 52s
specs (ubuntu, 3.4.7, --parser=parse.y)	1m 41s
specs (ubuntu, 3.4.7, --enable-frozen-string-literal)	1m 36s
	25m 45s

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