I don't believe there is any racial intent in the statements Jem and Dill made about striking a match under a turtle. The remarks came about following Dill's suggestion that they try and make Boo come out of his house. Dill then dared Jem to "put your big toe in the front yard," betting his copy of The Gray Ghost that Jem was too scared to do it. Jem warned Dill that Boo would "kill us each and every one" and gouge Dill's eyes out. Jem thought deeply about some way they could bring Boo out without endangering themselves, and he told Dill that
"... you have to think about these things... it's sort of like making a turtle come out..."
When Dill asked what he meant, Jem told him that you
"Strike a match under him."
Jem's point is that striking a match under a turtle who has hidden his head within his shell will force the turtle to stick out his head. They need a similar plan to coax Boo outside his house. Jem and Dill then debate about whether a turtle feels pain, and this seems to symbolically foreshadow the effects of their later harrassment of Boo, and whether Boo feels pain--like the turtle--from their efforts.