The next commit will use "echo -e". The problem with that is that bash
and sh differ:

- in bash: this results in "a\nb" (a+newline+b)
- in sh: this results in -e a\nb (-e a+newline+b)

The problem is that by default, commands are executed through sh, which
nobody expects. Change to bash, which is likely more aligned with what
people want to use.

Add some basic unit tests to verify that Iperf/Ping work. Harmonize the
logging by only print in the "main" iperf/ping function, to avoid that
if we test with multiple UEs, the output intermixes. Also, do one common
mkdir before starting multiple pings/iperfs. Finally, harmonize the
paths so that Iperf and Ping log their files in the same place.

- Return code says if copying succeeded
- Improve handling of the recursive code paths, which copy files or
  directories recursively into another directory
- both for RemoteCmd/LocalCmd

- allow executing in background
- less logs
- no use of split()
- common implementation of legacy command()

Signed-off-by: Raphael Defosseux <raphael.defosseux@eurecom.fr>

Signed-off-by: Raphael Defosseux <raphael.defosseux@eurecom.fr>