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 <bbruce@ucdavis.edu>
Reviewed-by: Jason Lowe-Power <power.jg@gmail.com>
Maintainer: Bobby R. Bruce <bbruce@ucdavis.edu>
Tested-by: kokoro <noreply+kokoro@google.com>

tests/gem5/insttest_se/test.py

@@ -40,7 +40,7 @@ else:
 
 for isa in test_progs:
     for binary in test_progs[isa]:
-        ref_path = joinpath(getcwd(), "ref", binary)
+        ref_path = joinpath(getcwd(), "ref")
         verifiers = (
             verifier.MatchStdoutNoPerf(joinpath(ref_path, "simout")),
         )