doc: dev-tools: kselftest.rst: update config file location

Config fragment files should be placed in
tools/testing/selftests/<testdir>/config

Signed-off-by: Anders Roxell <anders.roxell@linaro.org>
Acked-by: Shuah Khan (Samsung OSG) <shuah@kernel.org>
Signed-off-by: Jonathan Corbet <corbet@lwn.net>