ext4: add .kunitconfig fragment to enable ext4-specific tests

As of [1], we no longer want EXT4_KUNIT_TESTS and others to `select`
their deps. This means it can get harder to get all the right things
selected as we gain more tests w/ more deps over time.

This patch (and [2]) proposes we store kunitconfig fragments in-tree to
represent sets of tests. (N.B. right now we only have one ext4 test).

There's still a discussion to be had about how to have a hierarchy of
these files (e.g. if one wanted to test all of fs/, not just fs/ext4).

But this fragment would likely be a leaf node and isn't blocked on
deciding if we want `import` statements and the like.

Usage
=====

Before [2] (on its way to being merged):
  $ cp fs/ext4/.kunitconfig .kunit/
  $ ./tools/testing/kunit/kunit.py run

After [2]:
  $ ./tools/testing/kunit/kunit.py run --kunitconfig=fs/ext4/.kunitconfig

".kunitconfig" vs "kunitconfig"
===============================

See also: commit 14ee5cfd4512 ("kunit: Rename 'kunitconfig' to '.kunitconfig'").
* The bit about .gitignore exluding it by default is now a con, however.
* But there are a lot of directories with files that begin with "k" and
  so this could cause some annoyance w/ tab completion*
* This is the name kunit.py expects right now, so some people are used
  to .kunitconfig over "kunitconfig"

[1] https://lore.kernel.org/linux-ext4/20210122110234.2825685-1-geert@linux-m68k.org/
[2] https://git.kernel.org/pub/scm/linux/kernel/git/shuah/linux-kselftest.git/commit/?h=kunit&id=243180f5924ed27ea417db39feb7f9691777688e

* 372/5556 directories isn't too much, but still not a small number:
$ find -type f -name 'k*' | xargs dirname | sort -u | wc -l
372

Signed-off-by: Daniel Latypov <dlatypov@google.com>
Link: https://lore.kernel.org/r/20210210013206.136227-1-dlatypov@google.com
Signed-off-by: Theodore Ts'o <tytso@mit.edu>

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