Fix test with shared zend_test (#16705)

Fix GH-16628: FPM logs are getting corrupted with this log statement

zlog_buf_prefix() can return a larger length than what actually was
written due to its use of snprintf(). The code in

Fix GH-16628: FPM logs are getting corrupted with this log statement

zlog_buf_prefix() can return a larger length than what actually was
written due to its use of snprintf(). The code in
zlog_stream_prefix_ex() does not take this into account, other callers
do. What ends up happening then is that stream->length is set to the
length as if snprintf() was able to write all bytes, causing
stream->length to become larger than stream->buf.size, causing a
segfault.

In case the buffer was too small we try with a larger buffer up to a
limit of zlog_limit. This makes sure that the stream length will remain
bounded by the buffer size.

This also adds assertions to make the programmer intent clear and catch
this more easily in debug builds.

Closes GH-16680.

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