Add manifest deletion benchmarks

Context

This MR adds benchmarks for manifest deletion during garbage collection.

We need these benchmarks so that we can compare upcoming performance improvements from !24 (merged) against the current implementation.

Rational

There are already some benchmarks that touch the manifest deletion logic (e.g. benchmarkMarkAndSweep), but we need to isolate it (i.e. the call to storage.Vacuum.RemoveManifest) to obtain more detailed and configurable benchmarks.

Note: I have extracted the functionality to create a test registry and repository from testutils.DriverSuite.benchmarkMarkAndSweep so that it can be reused. I noticed that similar functions already exist in registry/storage/garbagecollect_test.go but they are unexported. Exporting these test utility functions would pollute the storage package. Extracting these to the testutil package is not possible either because it would cause an import cycle.