tests: Fix gem5 testlib compilation (#1063)

Prior to this patch the usage of KConfig was creating an empty config in
the case where a protocol was not specified.

tests/gem5/fixture.py

@@ -171,7 +171,7 @@ class SConsFixture(UniqueFixture):
                 "You may want to use --skip-build, or use 'rerun'."
             )
 
-        if self.protocol:
+        # Create the KConfig configuration based on the ISA
         defconfig_command = [
             "scons",
             "-C",
@@ -182,6 +182,11 @@ class SConsFixture(UniqueFixture):
             self.target_dir,
             joinpath(self.directory, "build_opts", self.isa.upper()),
         ]
+        log_call(log.test_log, defconfig_command, time=None, stderr=sys.stderr)
+
+        # If there is a cache coherence protocol specified,
+        # set it to the config.
+        if self.protocol:
             setconfig_command = [
                 "scons",
                 "-C",
@@ -192,13 +197,12 @@ class SConsFixture(UniqueFixture):
                 self.target_dir,
                 f"RUBY_PROTOCOL_{self.protocol.upper()}=y",
             ]
-            log_call(
-                log.test_log, defconfig_command, time=None, stderr=sys.stderr
-            )
             log_call(
                 log.test_log, setconfig_command, time=None, stderr=sys.stderr
             )
 
+        # Ensure the test objects are compiled into the binary by
+        # setting it in the config.
         setconfig_add_test_obj_command = [
             "scons",
             "-C",