Following files: e2e test, golden screens, stress benchmark

An e2e test appends to a CSV file while 012 follows it: new rows,
a partial line waiting, a formula recalculating, truncation, pausing
and unlinking. Golden screens show a followed log with its tab mark
and a formula over it, the question of how many rows to keep and a
linked file that isn't there, dark and light. BenchmarkFollow appends
10,000 rows a second for three seconds and times each update applied
and each frame.

An error op no longer counts as an update of the region, so its time
shows when rows last arrived; the state is shown without monochrome
color too.

commit

766b432fe975b2bc5e417013efe3e7190e9518c2

parent

e29c1ef868d2cc17b091edd1a8efcd4edbb9dcd0

author

Dami <dami.etoile@gmail.com>

kind

test risk score 1.7 of 3, Jev's estimate