futhark test should output unflattened indices

[Munksgaard]

Right now, when running futhark test, the mismatch indices are reported in the flattened representation. For instance:

$ futhark test --backend=opencl --no-tuning nw-blocked.fut
nw-blocked.fut:
Compiling with --backend=opencl:
Running compiled program:
Running ./nw-blocked:
Entry point: main; dataset: data/large.in:
nw-blocked.fut.main.0.actual and nw-blocked.fut.main.0.expected do not match:
Value (0,789762) expected -5672i32, got -5673i32
...and 358918 other mismatches.

The size of the input is 1537x1537, so the reported index 789762 actually corrensponds to (513, 1281), and should be reported as such.