function updateBucketVFormat(bucketName, vFormat) {
            const db = mongoClient.db('metadata');
            return db.collection('__metastore')
                .updateOne({
                    _id: bucketName,
                }, {
                    $set: { vFormat },
                }, {});
        }
These tests are intended to see if the vFormat of buckets is respected
when performing operations on the buckets. We perform operations on
buckets with mixed vFormats that were created at the same time

The mongo metadata uses a cache to store the vFormat of buckets,
that value is supposed to be immutable, hence the value in cache never
gets updated. Also the value of the default vFormat to use can not be
modified while cloudserver is running.

To overcome these issues, we set the METADATA_MAX_CACHED_BUCKETS
environement variable to 1 which means that only one bucket vFormat is cached
at once, and set the DEFAULT_BUCKET_KEY_FORMAT variable to v1.

Now that we can only store one bucket vFormat in cache, buckets v0 and v1 are
created in order, which leads to the first bucket's (v0) vFormat being removed
from the cache. The vFormat of the v0 bucket is then updated manually.
Next time the v0 bucket vFormat gets requested it will return the updated version
i.e v0