tests: add noop to fixed-benchmark

I was just curious of how much the looping takes time without
conversion, so I added this.

My results on Intel(R) Core(TM) i5-2410M CPU @ 2.30GHz:

benchmarked noop:	1.876349827s
benchmarked magic:	2.245844470s
benchmarked div:	12.709085309s
benchmarked mul:	7.504838141s

Mul seems to take 15x the time magic does, cool!

Btw. the simple default cast of int32_t to double is slower than magic
for me, hence the use of union.

Signed-off-by: Pekka Paalanen <ppaalanen@gmail.com>