[scudo][standalone] Enabled SCUDO_DEBUG for tests + fixes

Summary:
`SCUDO_DEBUG` was not enabled for unit tests, meaning the `DCHECK`s
were never tripped. While turning this on, I discovered that a few
of those not-exercised checks were actually wrong. This CL addresses
those incorrect checks.

Not that to work in tests `CHECK_IMPL` has to explicitely use the
`scudo` namespace. Also changes a C cast to a C++ cast.

Reviewers: hctim, pcc, cferris, eugenis, vitalybuka

Subscribers: mgorny, #sanitizers, llvm-commits

Tags: #sanitizers, #llvm

Differential Revision: https://reviews.llvm.org/D70276