rusty-x86: Unicorn engine tests are flaky

[DCNick3]

These problems have risen after migration from unicorn to unicorn-engine crate, which itself was prompted by ABI-breaking update of 2.0.1 of unicorn itself:

I think there is some thread synchronization issue in the unicorn-engine (or LLVM, which was also updated!), as some tests are non-deterministically failing with multiple threads but are fine in single-threaded context.

Then there is a problem of the pushfd_pop_eax test, which simply does

pushf
pop eax

The mismatch is that rusty-x86 has EAX be 0, while unicorn has it be 2. This result is stable, but I don't think it's right