Revert "[lit] Remove dead code not referenced in the LLVM SVN repo."

Summary:
This reverts rL306623, which removed `FileBasedTest`, an abstract base class,
but did not also remove the usages of that class in the lit unit tests.
The revert fixes four test failures in the lit unit test suite.

Test plan:
As per the instructions in `utils/lit/README.txt`, run the lit unit
test suite:

```
utils/lit/lit.py \
    --path /path/to/your/llvm/build/bin \
    utils/lit/tests
```

Verify that the following tests fail before applying this patch, and
pass once the patch is applied:

```
lit :: test-data.py
lit :: test-output.py
lit :: xunit-output.py
```

In addition, run `check-llvm` to make sure the existing LLVM test suite
executes normally.

Reviewers: george.karpenkov, mgorny, dlj

Reviewed By: mgorny

Subscribers: llvm-commits

Differential Revision: https://reviews.llvm.org/D35877

llvm-svn: 309120