tests: Fix incorrect ref path for insttest
tests: Fix incorrect ref path for insttest

This is part of the fix for the following nightly build error:
https://www.mail-archive.com/gem5-dev@gem5.org/msg40426.html

Change-Id: I60d9052b5d6dc812892b3d90baf19b8fd396f47b
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5/+/50228
Reviewed-by: Bobby R. Bruce <b9dba908ffb6c6d8ffe449cc88b3709c13c485c2@ucdavis.edu>
Reviewed-by: Jason Lowe-Power <92de577f37050ad05b60626d2f17913f40dcb1a2@gmail.com>
Maintainer: Bobby R. Bruce <b9dba908ffb6c6d8ffe449cc88b3709c13c485c2@ucdavis.edu>
Tested-by: kokoro <2ac7b1f3fa578934c95181d4272be0d3bca00121@google.com>

gem5/gem5,gem5/gem5,gem5/gem5,gem5/gem5,gem5/gem5,gem5/gem5,gem5/gem5