[LLDB] Remove undefined behavior in TestConstStaticIntegralMember.py

Setting an enum without a fixed underlying type to a value which is outside the
value range is undefined behavior.

The initializer needs to be a constant expression and therefore this was always
ill-formed we just were not diagnosing it before.

See D130058 and D131307 for more details.

Differential Revision: https://reviews.llvm.org/D131460