x86: add tests to cover defaulting of operand sizes for ambiguous insns

x86: add tests to cover defaulting of operand sizes for ambiguous insns

Prior to changing the logic in the assembler install tests to make sure
the present defaulting of operand sizes won't get broken. There are a
few anomalies pointed out by this:
- arithmetic insns (add, sub, etc) allow defaulting when their immediate
  fits in (signed) 8 bits, but they fail to assemble with larger values,
- mov, other than arithmetic insns, doesn't allow any defaulting,
- movsx/movzx default to byte sources (in AT&T mode), and their special
  casing needs to be adjusted first
- bt and friends allow defaulting, while shl and friends don't,
- ambiguous AVX and AVX512 insns don't allow defaulting.
This should ultimately all become consistent (perhaps with the exception
some of the SIMD insns); respective tests will be added to the test
cases here as the issues get addressed.