tests: There is no architecture called "timing".

I'm sure that's supposed to be "x86". By switching it over, the x86
regression tests became runnable again.

Change-Id: I9505703a0be71047ef3dd312ae83e76c2b32fdb5
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5/+/18568
Reviewed-by: Daniel Carvalho <odanrc@yahoo.com.br>
Reviewed-by: Jason Lowe-Power <jason@lowepower.com>
Reviewed-by: Anthony Gutierrez <anthony.gutierrez@amd.com>
Maintainer: Gabe Black <gabeblack@google.com>
Tested-by: kokoro <noreply+kokoro@google.com>

tests/testing/tests.py

@@ -126,7 +126,7 @@ arch_configs = {
         't1000-simple-x86',
     ),
 
-    ("timing", None) : (
+    ("x86", None) : (
         'pc-simple-atomic',
         'pc-simple-timing',
         'pc-o3-timing',