[scudo] Ensure proper allocator alignment in TSD test

The `MockAllocator` used in `ScudoTSDTest` wasn't allocated
properly aligned, which resulted in the `TSDs` of the shared
registry not being aligned either. This lead to some failures
like: https://reviews.llvm.org/D103119#2822008

This changes how the `MockAllocator` is allocated, same as
Vitaly did in the combined tests, properly aligning it, which
results in the `TSDs` being aligned as well.

Add a `DCHECK` in the shared registry to check that it is.

Differential Revision: https://reviews.llvm.org/D104402