qa: Remove unnecessary global statements in tests

qa: Remove unnecessary global statements in tests

Removes unused `global` statements from Python test files to fix flake8
F824 errors.

Recent flake8 runs were failing with:

```
./tasks/radosgw_admin.py:330:5: F824 `global log` is unused: name is never assigned in scope
./workunits/dencoder/test_readable.py:99:5: F824 `global incompat_paths` is unused: name is never assigned in scope
./workunits/dencoder/test_readable.py:164:5: F824 `global backward_compat` is unused: name is never assigned in scope
./workunits/dencoder/test_readable.py:165:5: F824 `global fast_shouldnt_skip` is unused: name is never assigned in scope
```

Since these variables are only being referenced and not assigned within
their scopes, the `global` declarations are unnecessary and can be
safely removed. This change:

- Removes all flagged `global` statements
- Fixes the failing flake8 checks in the CI pipeline
- Maintains the original code behavior as variable references still work
  without the `global` keyword

The `global` keyword is only needed when assigning to global variables
within a function scope, not when simply referencing them.

Signed-off-by: Kefu Chai <tchaikov@gmail.com>
(cherry picked from commit bcc275f98cb26c9432c320cd24ee93ff629e0574)