selftests/intel_pstate: check if cpupower is installed

Running "make kselftest TARGETS=intel_pstate" results in the
following errors:
- ./run.sh: line 89: cpupower: command not found
- ./run.sh: line 91: cpupower: command not found
if the cpupower is not installed.

Since the test depends on cpupower, this patch stops the test if the
cpupower is not installed.

Link: https://lore.kernel.org/all/cc01753c8dab0f33669a5a0fc162544078055bd1.1730141362.git.alessandro.zanni87@gmail.com/


Signed-off-by: Alessandro Zanni <alessandro.zanni87@gmail.com>
Signed-off-by: Shuah Khan <skhan@linuxfoundation.org>