test: fix long_row_timeout.test.lua failure in parallel mode

The test used to write big rows (20 mb in size), so when run in parallel
mode, it put high load on the disk and processor, which made appliers
time out multiple times during read, and caused the test to fail
occasionally.
So, instead of writing huge rows in test, introduce a new error
injection restricting sio from reading more than a couple of bytes per
request. This ensures that the test is still relevant and makes it a lot
more lightweight.

Closes #4062