Invalid memory operation during allocation with `new`

[Per Nordlöw]

My application has suddendly started crashing as

core.exception.InvalidMemoryOperationError at src/core/exception.d(647): Invalid memory operation

during a plain allocation with `new` during execution of `main`.

The crash happens only if the `unittests` are run prior to `main`.

The crash goes away when I disable at least on the `unittest`s 
run before `main`.

I'm compiling with DMD version 2.090.

How do I check the amount of memory currently being used in a 
process?

If the amount of memory used is not the problem, how can I find 
the reason for this?