tests,ext: Add skip_cleanup implementation for TempdirFixture

Change-Id: Idc5ec9309a4ef3c0ad0c7e8b2df47294acc97ec4
Signed-off-by: Jason Lowe-Power <jason@lowepower.com>
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5/+/17451
Reviewed-by: Andreas Sandberg <andreas.sandberg@arm.com>

ext/testlib/fixture.py

@@ -98,6 +98,13 @@ class Fixture(object):
     def copy(self):
         return copy.deepcopy(self)
 
+    def skip_cleanup(self):
+        '''
+        If this method is called, then we should make sure that nothing is
+        done when the teardown() function is called.
+        '''
+        pass
+
 
 def globalfixture(fixture):
     '''