None of the answers previously presented here address the part "without braking your system".
If you look at the packages installed in Raspbian, you will notice that some of them come from a collection that are essentially the Debian package collection ported to armv6. However, another part of the packages are the Foundation's own created packages that come from another collection.
If you go and mix distributions by adding packages from testing with these other packages, there is a possibility that you end up in a situation where dependencies cannot be met. So, some package will not be able to install any version, or you might not be able to remove a package or whatever. In addition to that Debian's testing packages do contain more bugs and change quite rapidly (so the possibility of breakage is always eminent, even if it works now).
Testing packages also do not get security patches and backports from the security team, although the actual package managers might incorporate such updates into their own code.
Because of all of this, it is clearly better to use the older packages in the official repo, unless you have some packages that absolutely need to be of a newer version. In that case, it would be much more safer to backport only those packages by building the source against the official distribution's packages (if someone else has not already done it) than dist-upgrade to stretch/testing.