Running tests for a large library on Windows

[pineapple]

I do mach.d - https://github.com/pineapplemachine/mach.d

I've been setting up CI tests and OSX and Linux tests are good to 
go thanks to Travis. I'm having a little more difficulty with 
testing on Windows via AppVeyor, since DMD is too memory-hungry 
to `dub test` without a fatal error when DMD was compiled for a 
32-bit target as opposed to 64-bit, as is the case for all 
Windows releases.

How can I compile and run unit tests for only a part of the 
library at a time, so that 32-bit Windows can run tests too? 
(Which is actually important, since there are different x86 and 
x86-64 implementations of some library functions.) Or is there 
another solution? It's my understanding that phobos would have a 
similar problem if it didn't do something deliberate with how 
tests are run.