all: Modify tests not to spoil code coverage results

When checking the code coverage of functional tests (using 'gcov'), the results
were been spoiled by killing servers and clients on the end of each test. This
commit introduces an ability to kill them more gently using SIGUSR1 in a way
that doesn't affect the code coverage result.

Change-Id: Ib81143425e9bc79592b19af82fde6294d3e9db2d