Add support for multi-file tests (#76)

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

Author: lucic71

CMakeLists.txt

@@ -129,6 +129,7 @@ add_custom_target("check-unit"
                           ${LIT_FLAGS}
                           "${PROJECT_SOURCE_DIR}/tests/unit"
                           "${PROJECT_SOURCE_DIR}/tests/ub"
+                          "${PROJECT_SOURCE_DIR}/tests/multi-file"
                   DEPENDS cpp2rust libcc2rs
                   USES_TERMINAL
                  )