Test: clear: local, streaming, live, tracefile rotation

This is the base testsuite for the lttng clear command.

The following scenarios are validated:

    local
    local with tracefilei rotation
    streaming
    streaming with tracefile rotation
    live with viewer attach while clear is issued
    live with viewer attach after clear is issued
    live with viewer attach while clear is issued and tracefile rotation active

These tests are performed for kernel and userspace domain.

When testing in userspace the following scenarios are also tested:
     Per pid and clear: this should fail. It is not supported yet.
     relayd disallow clear: clear must fail when relayd does not allow it

For the test using a viewer a testpoint was added to lttng-relayd live
protocol to give a synchronization point between babeltrace and the test
script.

The common_test script is a bit convoluted due to the reuse of the same
script for both ust and kernel testing. The alternative is to have
almost the same code (90%) in two separate files.

Signed-off-by: Jonathan Rajotte <jonathan.rajotte-julien@efficios.com>