test: fix strtod() test

test: fix strtod() test

One strtod() test is broken since:

commit 8e211000025940b770794abf5754de61b4add0af
Author: Thomas Hindoe Paaboel Andersen <phomes@gmail.com>
Date:   Mon Aug 4 23:13:31 2014 +0200

    test: use fabs on doubles

The commit was right, so no reason to revert it, but the test was broken
before and only worked by coincidence. Convert "0,5" to "0.5" so we don't
depend on locales for double conversion (or well, we depend on "C" which
seems reasonable).