Add a16w8 per-op test for bmm #5288

Job	Run time
test-xnnpack / test-backend-linux (xnnpack, models) / linux-job	23m 44s
test-xnnpack / test-backend-linux (xnnpack, operators) / linux-job	15m 11s
test-xnnpack / test-backend-macos	0s
test-xnnpack / package-golden-artifacts	3m 56s
	42m 51s

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