Storage: un-skip system tests for 'Bucket.generate_signed_url'

[tseaver]

Follow-on from PR #7460.

Currently, the GCS back-end has a bug for signed URLs against buckets: unlike blobs, which expect an appropriately-capitalized X-Goog-Signature query parameter, buckets want that parameter lower-cased. The system tests for signed bucket URLs are thus skipped in the PR, which is likely to merge before a back-end fix rolls out.

Once the fix is rolled up, remove the skips.

[tseaver]

/cc @frankyn

[frankyn]

This is unblocked and can now be unskipped.

[tseaver]

8f2e13f in PR #7460 removes the skips.