Stop bashing functest scripts

Stop bashing functest scripts

Though bash is a good interactive user shell
and apart portability concerns,
many UN*X systems and GNU+Linux distributions now prefers Almquist
or Kornshell variants for scripting in order to improve the system
performance and maintenability.

https://wiki.ubuntu.com/DashAsBinSh
https://mywiki.wooledge.org/Bashism
https://en.wikipedia.org/wiki/Almquist_shell

Signed-off-by: guillaume.lambert <guillaume.lambert@orange.com>
Change-Id: I32c5490f9eab54e6cac594f3eac1e392edf8b272
(cherry picked from commit b9fedb6868306a6e3145b7ee851ea3a84353df9c)