test-ellipsize: add tests for ellipsize_mem, fix bugs

First, ellipsize() and ellipsize_mem() should not read past the input
buffer. Those functions take an explicit length for the input data, so they
should not assume that the buffer is terminated by a nul.

Second, ellipsization was off in various cases where wide on multi-byte
characters were used.

We had some basic test for ellipsize(), but apparently it wasn't enough to
catch more serious cases.

Should fix https://bugs.chromium.org/p/oss-fuzz/issues/detail?id=8686.

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