Create a custom allocator for the `buf::oom` tests that will fail with
out-of-memory errors in predictable ways.  We were previously trying to
guess the way that various allocators on various platforms would fail
in a way such that `malloc`/`realloc` would return `NULL` (instead of
aborting the application, or appearing suspicious to various
instrumentation or static code analysis tools like valgrind.)

Introduce a fake `malloc` and `realloc` that will return `NULL` on
allocations requesting more than 100 bytes.  Otherwise, we proxy to the
default allocator.  (It's important to use the _default_ allocator, not
just call `malloc`, since the default allocator on Windows CI builds may
be the debugging C runtime allocators which would not be compatible with
a standard `malloc`.)