[lldb][test] TestConstStaticIntegralMember: relax assertion on number of global variables (#73707)

In https://github.com/llvm/llvm-project/pull/73626 we started attaching
`DW_AT_const_value`s on a static data-member's declaration again. In
DWARFv5, those static members are represented with a `DW_TAG_variable`.
When LLDB builds the `ManualDWARFIndex`, it simply iterates over all
DIEs in a CU and puts *any* `DW_TAG_variable` with a constant or
location into the index. So when using the manual index, we can end up
having 2 entries for a static data member in the index, one for the
declaration and one for the definition.

This caused a test failure on Linux (where DWARFv5 is the default and
the tests use the manual index).

This patch loosens the restriction that we find exactly 1 variable.