Don't swallow AttributeError from super().setUp()

This is conceptually independent of the immediately preceding
super() call refactoring, but serves the same goal of simplifying
and clarifying super() calls.

In test.performance.lib, the TestBigRepoR and TestBigRepoRW
classes' setUp methods had calls to setUp through super proxies,
which were wrapped in try-blocks to swallow AttributeError
exceptions. This removes that, relying on the presence of setUp
methods in some parent or sibling class in the MRO.

The intent appeared to be solely to account for the possibility
that no class in the MRO would define a setUp method. However,
the unittest.TestCase base class defines noop setUp and tearDown
methods to ensure this does not have to be done, allowing the
code to be simplified by integrating the assumption of their
presence.

This may also make the code more robust, because the form in which
AttributeError was being swallowed was:

    try:
        super().setUp()
    except AttributeError:
        pass

But that has the disadvantage of also catching AttributeError due
to a bug or other problem in code that runs *in* an ancestor or
sibling class's existing setUp method. This could alternatively
be addressed by using:

    try:
        other_setUp = super().setUp
    except AttributeError:
        pass
    else:
        other_setUp()

However, because unittest.TestCase provides empty setUp and
tearDown methods to allow such special-casing to be avoided (both
in cases like this and for the test runner), this isn't needed.

test/performance/lib.py

@@ -32,10 +32,7 @@ class TestBigRepoR(TestBase):
     """
 
     def setUp(self):
-        try:
-            super().setUp()
-        except AttributeError:
-            pass
+        super().setUp()
 
         repo_path = os.environ.get(k_env_git_repo)
         if repo_path is None:
@@ -64,10 +61,7 @@ class TestBigRepoRW(TestBigRepoR):
 
     def setUp(self):
         self.gitrwrepo = None
-        try:
-            super().setUp()
-        except AttributeError:
-            pass
+        super().setUp()
         dirname = tempfile.mktemp()
         os.mkdir(dirname)
         self.gitrwrepo = self.gitrorepo.clone(dirname, shared=True, bare=True, odbt=GitCmdObjectDB)