jjb: depends-on: do not run tests of dependencies

jjb: depends-on: do not run tests of dependencies

The important part is how lttng-tools behave. Running the tests of
the dependencies is redundant and only leads to wasted time and CPU time.

The RUN_TESTS flag is now "namespaced" to ensure that we can turn on and
off tests for each projects if necessary.

LTTNG_TOOLS_RUN_TESTS
LTTNG_TOOLS_RUN_TESTS_LONG_REGRESSION
LTTNG_UST_RUN_TESTS
USERSPACE_RCU_RUN_TESTS

For now only `LTTNG_UST_RUN_TESTS` and `USERSPACE_RCU_RUN_TESTS` can be
manipulated via environment injection. The lttng-tools build script
is a bit more complex and manipulates the value themselves. More work
will be required if we want to control the flags of the lttng-tools
build script.

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