Disable clang-format entirely for test directories

See discussion here:

https://github.com/llvm/llvm-project/issues/55982

And the RFC here:
https://discourse.llvm.org/t/rfc-disable-clang-format-in-the-clang-test-tree/63498/2

We don't generally expect test files to be formatted according to the
style guide. Indeed, some tests may require specific formatting for the
purposes of the test.

When tests intentionally do not conform to the "correct" formatting,
this causes errors in the CI, which can drown out real errors and causes
people to stop trusting the CI over time.

From the history of the clang/test/.clang-format file, it looks as if
there have been attempts to make clang-format do a subset of formatting
that would be useful for tests. However, it looks as if it's hard to
make clang-format do exactly the right thing -- see the back-and-forth
between
13316a77
and
7b5bddfd.

These changes disable the .clang-format file for clang/test, llvm/test,
and clang-tools-extra/test.

Fixes #55982
Differential Revision: https://reviews.llvm.org/D128706