selftests: sync: Skip the test if kernel support is not found

The "Sync framework" test doesn't work if the kernel has no support,
obviously. Rather than reporting a failure, check for the kernel support
by looking for /sys/kernel/debug/sync/sw_sync, and if not found skip the
test.

Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Reviewed-by: Gustavo Padovan <gustavo.padovan@collabora.com>
Signed-off-by: Shuah Khan <shuahkh@osg.samsung.com>