tests: Add 'checkpoint-path' to simpoints stdlib example

The
'configs/example/gem5_library_checkpoints/simpoints-se-checkpoint.py'
example would dump the savepoint in the CWD. This is fine when running
as an example, but we also run this script as a test. In this case the
checkpoint litters the repository.

To fix this, an optional 'checkpoint-path' argument is added to this
example which specifies where the checkpoint is to be saved. In the
tests, the checkpoint is saved to 'tests/gem5/resources'. This is our
default location for resources needed/produced by tests and is ignored
by git.

Change-Id: I3dccc574b9e64d32386fd822ed7248ee365a0a08
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5/+/64092
Maintainer: Jason Lowe-Power <power.jg@gmail.com>
Reviewed-by: Jason Lowe-Power <power.jg@gmail.com>
Tested-by: kokoro <noreply+kokoro@google.com>
2 files changed