test: fix strtod() test

One strtod() test is broken since:

commit 8e21100002
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).

src/test/test-util.c

@@ -212,7 +212,7 @@ static void test_safe_atod(void) {
         assert_se(r == -EINVAL);
 
         errno = 0;
-        assert_se(fabs(strtod("0,5", &e) - 0.5) < 0.00001);
+        assert_se(fabs(strtod("0.5", &e) - 0.5) < 0.00001);
 
         /* And check again, reset */
         setlocale(LC_NUMERIC, "C");