tests: Update testing documentation

This edits the documentation regarding the usage of the --isa
tag, as this has fallen out of date in regards to the new
'ALL' isa.

Change-Id: I3b672ac2c03dd109bba458db688af05ed4135a91
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5/+/65651
Tested-by: kokoro <noreply+kokoro@google.com>
Reviewed-by: Bobby Bruce <bbruce@ucdavis.edu>
Maintainer: Bobby Bruce <bbruce@ucdavis.edu>

tests/gem5/fixture.py

@@ -174,8 +174,7 @@ class SConsFixture(UniqueFixture):
             )
             log.test_log.message("%s" % (", ".join(self.targets)))
             log.test_log.message(
-                "You may want to run with only a single ISA"
-                "(--isa=), use --skip-build, or use 'rerun'."
+                "You may want to use --skip-build, or use 'rerun'."
             )
 
         command.extend(self.targets)