[clang][dataflow] Refactor `clang/Analysis/FlowSensitive/MatchSwitchTest.cpp`.

- Remove use of `runDataflowAnalysis` to keep test isolated.
- Add test for `ASTMatchSwitch<CXXCtorInitializer, ...>`.

Reviewed By: gribozavr2, sgatev

Differential Revision: https://reviews.llvm.org/D133935