lttng-tools: skip kernel tests if no kernel modules present

lttng-tools: skip kernel tests if no kernel modules present

The current tests will run both userspace and kernel testing. Some of use cases
only use lttng for one kind of tracing (e.g. userspace). If the lttng
modules(.ko files) is not present during the test,it would end up with lots of
failing.

Add a check in ptest script, if current system doesn't contain lttng kernel
modules, passing LTTNG_TOOLS_DISABLE_KERNEL_TESTS=1 to make to skip all lttng
kernel related testing.

Signed-off-by: Xiangyu Chen <xiangyu.chen@windriver.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>