x86: rework noavx512-1 testcase

x86: rework noavx512-1 testcase

So far the set of ".noavx512*" has been accumulating, which isn't ideal.
In particular this hides issues with dependencies between features.
Switch back to the default ISA before disabling a particular subset.
Furthermore limit redundancy by wrapping the repeated block of insns in
an .irp.