Fix a for loop bug introduced in the concurrency refactor

Fix a for loop bug introduced in the concurrency refactor

Due to Go's idiosyncratic semantics of for loops, tests would only run
some of the test groups in the JSON file because by the time the closure
is called, the array index could be changed. For example, on fast 8 core
machines, the CMAC tests would run the last test group with key size 320
eight times rather than each of the eight test groups once.

Make a copy of the pointer before passing it to the closure to avoid this
issue.

Simpler version of my initial fix from jsing