ALGORITHMS: Fix a few remaining differences between DFT tests

Signed-off-by: Gama <misept.dieseneuf@gmail.com>
ALGORITHMS: Fix DFT test on sin finding

Since the output data is fully complex, but on a real signal, issues
to find the actual sinus peak may occur.
Hence the search for the extremum is limited to half the output.

Signed-off-by: Gama <misept.dieseneuf@gmail.com>