Move test/.gitattributes to clang-tools-extra/test

It was probably accidentally added there, see the discussion on change
D97625. It is certainly without effect, to quote gitattributes(5):

    When deciding what attributes are assigned to a path, Git consults
    [...], `.gitattributes` file in the same directory as the path in
    question, and its parent directories up to the toplevel of the work
    tree [...]

Running `git check-attr -a` on the files in question shows that now the
settings are indeed effective whereas before they were not.

Lastly, lit ignores the file like any dotfile, see getTestsInDirectory
of FileBasedTest in llvm/utils/lit/lit/formats/base.py. This can be
verified with `llvm-lit --show-tests clang-tools-extra/test`.