test_runner: fix escaping in snapshot tests

Snapshotted values are escaped after serialization. This happens when serializing a value for comparison when snapshots already exist, and also when updating them. That is, snapshots are escaped in the internal storage, but when written to disk, one "level" of escaping is removed. That escaping is never added back when reading the snapshots back. This makes even the simplest test trying to serialize a string with an escape code in it fail, like the one I added here.

Honestly I'm not quite sure about the approach here: An alternative would be to not escape snapshots internally, and only do so when writing them to disk/interpolating them into the appropriate JS-code. I haven't fully thought this through, though, and there is also a test explicitly ensuring that serialize escapes values, so there might be a good reason for it.

Another thing I'm not sure about is whether the test I added is sufficient, because it feels kind of indirect. Let me know if you want to see that changed.