test: fix undefined order of functio parameters.

[trofi]

Detected instability when built flatbuffers-2.0.0 on gcc-12:

[ 75%] Building CXX object CMakeFiles/flattests.dir/tests/test_builder.cpp.o
.../c++/12.0.0/bits/shared_ptr_base.h:397:45: error: 'size' may be used uninitialized [-Werror=maybe-uninitialized]
  397 |       explicit _Sp_ebo_helper(_Tp&& __tp) : _M_tp(std::move(__tp)) { }
      |                                             ^~~~~~~~~~~~~~~~~~~~~~
In file included from flatbuffers/tests/test_builder.cpp:1:
flatbuffers/tests/test_builder.h: In function 'void builder_move_assign_after_releaseraw_test(Builder) [with Builder = flatbuffers::FlatBufferBuilder]':
flatbuffers/tests/test_builder.h:63:10: note: 'size' was declared here
   63 |   size_t size, offset;
      |          ^~~~
...
In file included from flatbuffers/tests/test_builder.cpp:1:
flatbuffers/tests/test_builder.h: In function 'void builder_move_assign_after_releaseraw_test(Builder) [with Builder = GrpcLikeMessageBuilder]':
flatbuffers/tests/test_builder.h:63:10: note: 'size' was declared here
   63 |   size_t size, offset;
      |          ^~~~
cc1plus: all warnings being treated as errors

Here is the relevant bit of test:

template<class Builder>
void builder_move_assign_after_releaseraw_test(Builder b1) {
  auto root_offset1 = populate1(b1);
  b1.Finish(root_offset1);
  size_t size, offset;
  std::shared_ptr<uint8_t> raw(
      b1.ReleaseRaw(size, offset), [size](uint8_t *ptr) {
        flatbuffers::DefaultAllocator::dealloc(ptr, size);
      });

Note how b1.ReleaseRaw(size, offset) is expected to populate size
and [size](uint8_t *ptr) { captures the result. But both are parameters
to the same function call and thus evaluation order is unspecified.

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Thanks!

[aardappel]

Yup that makes sense, thanks for fixing!