Fix non-deterministic process test

* test/src/process-tests.el (set-process-filter-t): Don't assume
subprocess output will come in a single chunk, keep waiting for more
data until next "prompt" is read from subprocess.

(cherry picked from commit aa49aa88)
1 job for emacs-26 in 25 minutes and 29 seconds (queued for 1 second)
Status Job ID Name Coverage
  Test
passed #3175
test

00:25:29