[lldb][test] Disable TestSBValueUnsignedEnumBitField.py for old DWARF versions

With older DWARF versions we don't encode the enum's underlying
type in DWARF. In those cases LLDB sign-extends the bitfield as
a signed integer. Without the actual enum type being present in
DWARF there's not much we can do.

Differential Revision: https://reviews.llvm.org/D134734