Make ./snappy_unittest pass without "srcdir" being defined.

Previously, snappy_unittests would read from an absolute path /testdata/..;
convert it to use a relative path instead.

Patch from Marc-Antonie Ruel.

R=maruel


git-svn-id: https://snappy.googlecode.com/svn/trunk@72 03e5f5b5-db94-4691-08a0-1a8bf15f6143