[test] [lldb] Use filename instead of index in test

In some environments this test could fail if start.S has its own DWARF
CompileUnit or similar are included before the DWARF CompileUnit for the
file.

This change makes the test independent of the index of the compile unit,
instead checking the filename.

Reviewed By: herhut, jankratochvil

Differential Revision: https://reviews.llvm.org/D107300