Add ability to read .wav files to test-resample

Previously test-resample could only read .raw files, and the input
sample rate had to be specified.  Now the input sample rate is derived
from the input file.  This also allows us to read 8-bit PCM files,
and other formats such as floating-point in the future.

However, the ability to read raw files is lost.
A workaround is to use sox or equivalent on the host.

Change-Id: Icd06b4d02482b3ad07bf03979f46860e68d38ad9