[test] Fix filter test data

Signed-off-by: Claudio Matsuoka <cmatsuoka@gmail.com>
Fix floating point values in lowpass filter

Don't convert carefully computed floating point values to int before
using. Ugh. Thanks to clang in travis-ci, we'll now use the floating
point values in all their glory.

Signed-off-by: Claudio Matsuoka <cmatsuoka@gmail.com>