DebugInfoD tests + fixing issues exposed by tests (#85693)

Finally getting back to Debuginfod tests:
I've migrated the tests in my [earlier
PR](https://github.com/llvm/llvm-project/pull/79181

) from shell to API
(at @JDevlieghere's suggestion) and addressed a couple issues that came
about during testing.

The tests first test the "normal" situation (no DebugInfoD involvement,
just normal debug files sitting around), then the "no debug info"
situation (to make sure the test is seeing failure properly), then it
tests to validate that when Debuginfod returns the symbols, things work
properly. This is duplicated for DWP/split-dwarf scenarios.

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Co-authored-by: Kevin Frei <freik@meta.com>