test/test_compression.sh : Fix for OSX.

On Mac OSX, the 'wc -c' command returns a number with leading whitespace
which then results in a syntax error when the shell parser expects a
number when it relly has a string containing a number. The easy fix is
to use string interpolation.

Closes: https://sourceforge.net/p/flac/bugs/420/
Reported-by: Mark Harris <mark.hsj@gmail.com>