tests: Adding --bin-path option to select tests bin directory

tests: Adding --bin-path option to select tests bin directory

So far lots of tests will download binaries inside the gem5 directory.
The path is also specific to the test being run.
This doesn't play well with an environment where gem5 is cloned from
scratch for every build, or if several gem5 are cloned in a single
machine.
Binaries will be automatically downloaded every time this happens.

This patch is adding a --bin-path option, so that it's possible to
setup a fixed directory with all pre-downloaded binaries.
By default it is set to None to preserve original behaviour.

Change-Id: I42fb25e3ce0a495c73672b15a097b1bd2607795c
Signed-off-by: Giacomo Travaglini <giacomo.travaglini@arm.com>
Reviewed-by: Ciro Santilli <ciro.santilli@arm.com>
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5/+/24525
Reviewed-by: Jason Lowe-Power <jason@lowepower.com>
Reviewed-by: Bobby R. Bruce <bbruce@ucdavis.edu>
Maintainer: Jason Lowe-Power <jason@lowepower.com>
Tested-by: kokoro <noreply+kokoro@google.com>