[gdb/tdep] Add vzeroupper and vzeroall in amd64-insn-decode selftest

After I posted a tentative patch for PR31952, Alexander Monakov pointed out
that the patch broke instruction decoding for instructions vzeroall and
vzeroupper.

Add selftests for these two instructions in amd64-insn-decode, both using
vex2 and vex3 prefixes.

Tested on x86_64-linux.