tests: Add base class for fixtures that generate a target file

tests: Add base class for fixtures that generate a target file

The new TargetFixture can be used as a base class for fixtures that
generate/download a file. These fixtures are guarrantied to be unique
and their setup function is only executed once.

Change-Id: I6a8737b06c4e74f3e29736ec363f61251d85da8c
Signed-off-by: Nikos Nikoleris <nikos.nikoleris@arm.com>
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5/+/19250
Reviewed-by: Giacomo Travaglini <giacomo.travaglini@arm.com>
Reviewed-by: Jason Lowe-Power <jason@lowepower.com>
Maintainer: Jason Lowe-Power <jason@lowepower.com>
Tested-by: kokoro <noreply+kokoro@google.com>