ext4: add kunit test for decoding extended timestamps

KUnit tests for decoding extended 64 bit timestamps that verify the
seconds part of [a/c/m] timestamps in ext4 inode structs are decoded
correctly.

Test data is derived from the table in the Inode Timestamps section of
Documentation/filesystems/ext4/inodes.rst.

KUnit tests run during boot and output the results to the debug log
in TAP format (http://testanything.org/

). Only useful for kernel devs
running KUnit test harness and are not for inclusion into a production
build.

Signed-off-by: Iurii Zaikin <yzaikin@google.com>
Reviewed-by: Theodore Ts'o <tytso@mit.edu>
Reviewed-by: Brendan Higgins <brendanhiggins@google.com>
Tested-by: Brendan Higgins <brendanhiggins@google.com>
Reviewed-by: Shuah Khan <skhan@linuxfoundation.org>
Signed-off-by: Shuah Khan <skhan@linuxfoundation.org>