tests: Add missing _pre_instantiate()

As of this change:
https://gem5-review.googlesource.com/c/public/gem5/+/65051, the
`_pre_instantiate` function must be called prior to `m5.instantiate`
when using the stdlib without the Simulator module. The
"test/gem5/replacement-policies/run_replacement_policy_test.py" lacked
this and was causing errors when running replacement policy tests.

In addition we also fix the incorrect type of size in`createArtifact'.
This was causing problems with the Kokoro build system.
The typing here was `int` but had a default value of `None`. The correct
type is therefore `Optional[int]`.

Change-Id: Ibaf63151196b15f68e643fa5c1b290439d6618c8
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5/+/66371
Maintainer: Bobby Bruce <bbruce@ucdavis.edu>
Tested-by: kokoro <noreply+kokoro@google.com>
Reviewed-by: Bobby Bruce <bbruce@ucdavis.edu>
(cherry picked from commit a3fd9631cc)
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5/+/66613

tests/gem5/replacement-policies/run_replacement_policy_test.py

@@ -85,6 +85,7 @@ motherboard = TestBoard(
 )
 root = Root(full_system=False, system=motherboard)
 
+motherboard._pre_instantiate()
 m5.instantiate()
 
 generator.start_traffic()

util/gem5art/artifact/gem5art/artifact/artifact.py

@@ -158,7 +158,7 @@ class Artifact:
         documentation: str,
         inputs: List["Artifact"] = [],
         architecture: str = "",
-        size: int = None,
+        size: Optional[int] = None,
         is_zipped: bool = False,
         md5sum: str = "",
         url: str = "",