[lld][WebAssembly] Prefer objdump -d over obj2yaml for tests. NFC

Now that we have https://reviews.llvm.org/D105539 we can
use objdump -d to actually check for instruction sequences
rather than binary blobs.

This is just an example of how to do that we should followup
with a wider ranging conversion of existing tests.

Differential Revision: https://reviews.llvm.org/D106897