[lldb][test] DWARFASTParserClangTests: extract test setup into helper structure (llvm#170132)

Depends on:
* llvm#170249

We keep repeating the boilerplate of creating a
`DWARFASTParserClangStub` and `TypeSystemClangHolder` in all the
unit-test cases. Lets extract this into a helper to make the tests
easier to grok.

We actually only need the `DWARFASTParserClangStub` and a
`TypeSystemClangHolder` in one of the test cases. For the rest, we can
just re-use the typesystem/parser that the `YAMLModuleTester` created.
Re-using them makes it more straightforward to write test-cases because
we don't need to worry about which TypeSystem which DWARFParser created
types into.

(cherry picked from commit 7685e1f)

Author: Michael137