tests: Standardized test resources download dir

tests: Standardized test resources download dir

We were downloading resources to various different locations, for no
real reason. This standardizes the process. From this commit onwards,
all testing resources are downloaded to `tests/gem5/resources` by
default. This may be overriden via the `--bin-path` TestLib argument.

Note: In order to do this I have changed the meaning of the `bin-path`
TestLib argument slightly. Previously the `bin-path` assumed a flat
(non-existant) hierarchy. A simple directory of local resources. This
new bin-path functionality maintains logical sub-directories. This is
technically an API change and will be noted in the release notes.

Change-Id: I4df85c121fa65f787fd71f03d74361afea121380
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5/+/33145
Reviewed-by: Hoa Nguyen <hoanguyen@ucdavis.edu>
Reviewed-by: Jason Lowe-Power <power.jg@gmail.com>
Tested-by: kokoro <noreply+kokoro@google.com>
Maintainer: Jason Lowe-Power <power.jg@gmail.com>