test: use yaml.safe_load() instead of yaml.load()

The primary reason why this change is needed is that yaml.load() w/o an
explicit loader was banned in Gentoo Linux for recent pyyaml versions;
see [1].

We don't use the pyyaml feature that allows to construct a Python object
based on a yaml tag, so safe_load() fit our needs.

See also related changes in test-run and tarantool-python ([2], [3], [4]).

[1]: https://gitweb.gentoo.org/repo/gentoo.git/commit/?id=79ba924d94cb0cf8559565178414c2a1d687b90c
[2]: https://github.com/tarantool/test-run/commit/38400e91c600677fb661154d00459d660fa9880d
[3]: https://github.com/tarantool/test-run/commit/89808d60eb3b5130e227fc1a7866f2ad5a197bea
[4]: https://github.com/tarantool/tarantool-python/commit/350771d240a18eec188a53e8c696028b41baa13f