selftests: timers: clocksource-switch: adapt to kselftest framework

So we have proper counters at the end of a test. We also print the
kselftest header at the end of the test, so we don't mix with the output
of the child process. There is only this one test anyhow.

Signed-off-by: Wolfram Sang <wsa+renesas@sang-engineering.com>
Acked-by: John Stultz <jstultz@google.com>
Signed-off-by: Shuah Khan <skhan@linuxfoundation.org>