autotest: explicitly remove /usr from coverage

The lcov --no-external option seems to still generate coverage data for
e.g. /usr/include/c++/4.8.2/tuple on some test pool machines, contrary to
the description of the option.  Add an explicit removal of all /usr file
coverage at the end of the test run to work around it.

Change-Id: I8579813b1b76b4bfdef86748e88352bf232df954
Signed-off-by: Daniel Verkamp <daniel.verkamp@intel.com>
Reviewed-on: https://review.gerrithub.io/369305


Tested-by: SPDK Automated Test System <sys_sgsw@intel.com>
Reviewed-by: Jim Harris <james.r.harris@intel.com>
Reviewed-by: Ben Walker <benjamin.walker@intel.com>