llvm-native: accelerate-nofib wrongly reports errors

[tvh]

When I run accelerate-nofib with +RTS -N4, It reports errors where there aren't any.

        square: [Failed]
*** Failed! Falsifiable (after 8 tests and 2 shrinks): 
Array (Z :. 1 :. 3) [1,-2,1]
*** Expected:
Array (Z :. 1 :. 3) [1,4,1]
*** Received:
Array (Z :. 1 :. 3) [1,4,1]

[tmcdonell]

Okay, so run itself is not thread-safe (which is annoying, but also why there is runWith), and we weren't correctly telling test-framework to only run a single test at a time for the LLVM CPU backend. The latter at least is fixed.