selftests: cpufreq: Write test output to stdout as well

Use 'tee' to send the test output to stdout in addition to the current
output file. This makes the output easier to handle in automated test
systems and is superior to only later dumping the output file contents
to stdout, since this way the test output can be interleaved with other
log messages, like from the kernel, so that chronology is preserved,
making it easier to detect issues.

Signed-off-by: Nícolas F. R. A. Prado <nfraprado@collabora.com>
Acked-by: Viresh Kumar <viresh.kumar@linaro.org>
Signed-off-by: Shuah Khan <skhan@linuxfoundation.org>