Alternating patterns in 8b/10b encoding
Alternating pattern for 8b10b encoding is D10.2 symbol. Check out table in wikipedia.

However this may not be what you want as for example gigabit Ethernet of the 1000BASE-T flavor uses PAM-5 modulation with data scrambling instead of NRZ with 8b10b encoding (again Wikipedia is helpful). 

Also end result will depend in which protocol layer you are injecting test data as different protocol layers will add/remove/modify data.