[vm/ffi] Write test for behavior of allocation of size-0 structs.

[sjindel-google]

Currently, it might succeed or throw an exception, depending on the behavior of malloc.

[dcharkes]

#37250 changes the Pointer.allocate() semantics to be consistent with malloc as well.

Semantics are malloc(sizeOf<T>() * count) where count and T are the argument and type argument passed to Pointer.allocate.

So I think the current semantics are fine.

We should probably add a test which is similar to https://dart-review.googlesource.com/c/sdk/+/107503/3/tests/ffi/data_test.dart

[sjindel-google]

I'm fine with the current semantics, as long as we understand how it's defined.

[sjindel-google]

Obsolete after #37265