[clang][dataflow] Replace TEST_F with TEST where possible

Many of our tests are currently written using `TEST_F` where the test fixture class doesn't have any `SetUp` or `TearDown` methods, and just one helper method. In those cases, this patch deletes the class and pulls its method out into a standalone function, using `TEST` instead of `TEST_F`.

There are still a few test files leftover in `clang/unittests/Analysis/FlowSensitive/` that use `TEST_F`:

- `DataflowAnalysisContextTest.cpp` because the class contains a `Context` field which is used
- `DataflowEnvironmentTest.cpp` because the class contains an `Environment` field which is used
- `SolverTest.cpp` because the class contains a `Vals` field which is used
- `TypeErasedDataflowAnalysisTest.cpp` because there are several different classes which all share the same method name

Reviewed By: ymandel, sgatev

Differential Revision: https://reviews.llvm.org/D128924