simpleperf: test kernel version in some feature tests.

Some features are only based on kernel support. So we can skip
the feature tests if we are on a high enough kernel version.
Also cache kernel version to speed up tests.

Bug: none
Test: run simpleperf_unit_test
Change-Id: Id163effba77b29f0fa57cb0feb4e8ea87884af0d
2 files changed