Add support for multi-file tests

[lucic71]

This PR adds support for writing multi translation-unit tests in the below format. All multi translation-unit tests go into tests/multi-file, one test per directory. test.expectations is an optional file that contains one of the following test expectations: no-compile, translation-fail, panic, nondet-result; or it can be empty if the test is successful:

tests/multi-file/
└── extern_functions
    ├── a.c
    ├── b.c
    ├── CMakeLists.txt
    ├── out
    │   ├── refcount
    │   │   └── extern_functions.rs
    │   └── unsafe
    │       └── extern_functions.rs
    └── test.expectations

I took this opportunity to refactor the logic inside Cpp2RustTest.py because the multi-file addition did not fit well into the current model. Now Cpp2RustTest.py is composed of:

• TestExpectations: dataclass describing the expected output of the test: should_panic, should_not_compile, should_not_translate, is_nondet_result, xfail. For classic tests (ub, benchmarks, unit-tests), TestExpectations is populated from the comment line inside the source file, for multi-file tests, TestExpectations is populated from test.expectatoins
• TestContext: dataclass that defines all test steps (setup, translate, check_expected, build_cpp, build_rust, run_cpp, run_rust, compare) and keeps test specific data as member variables, including TestExpectations
• Cpp2RustTest: entry point of lit, contains execute (which calls directly into TestContext) getTestsForPath and getTestsInDirectory
• free helper functions

[nunoplopes]

this is gone now

[lucic71]

Done

[nunoplopes]

this bit is gone now too

[lucic71]

Done