Tests: Move unit tests to tests/unit

The regression/tools/{test_kernel_data_trace,test_sessions,test_ust_data_trace}.c
and regression/tools/streaming/unit_tests.c tests files are really unit tests.

In order to have a clear separation between unit tests and integration/system tests,
move them to their own folder.

Signed-off-by: Christian Babeux <christian.babeux@efficios.com>