Fixes#24998
Helper routine translating input_stream buffers to single lines
did not loop over current buffer state, leading to only the first
line being sent to end listener.
Rewrote to use range iteration instead. Nicer.
Closesscylladb/scylladb#24999
Adds a wrapper for seastar::experimental::process, to help
use external process fixtures in unit test. Mainly to share
concepts such as line reading of stdout/err etc, and sync
the shutdown of these. Also adds a small path searcher to
find what you want to run.
