Skip copy_file_range based tests if running on older kernels

We use copy_file_range(2) in bolt_copy_bytes. It got added in kernel 4.5; glibc used to have fallback code, but that got removed with 2.30. Now if you are testing bolt via docker on hosts with older kernels (say 4.4) but in a container with recent glibc (2.30) the tests will break: launchpad bug 1843728 This adds a bunch of code to check the currently kernel version and skip the affected tests if the tests are run in quick mode.