tests/check-qjson: Don't skip funny QNumber to JSON conversions

tests/check-qjson: Don't skip funny QNumber to JSON conversions

simple_number() and float_number() convert from JSON to QNumber and
back.

simple_number() tests "-0", but skips the conversion back to JSON,
because it yields "0", not "-0".  Works as intended, so better cover
it: don't skip, but expect the funny result.

float_number() tests "-32.20e-10", but skips the conversion back to
JSON, because it yields "-0".  This is a known bug in
qnum_to_string(), marked FIXME there.  Cover the bug: don't skip, but
expect the funny result.

While there, switch from g_assert() to g_assert_cmpstr() & friends for
friendlier test failures.

Signed-off-by: Markus Armbruster <armbru@redhat.com>
Message-Id: <20201210161452.2813491-2-armbru@redhat.com>