selftest/functions: add two helper functions to measure the elapsed time

Signed-off-by: Olaf Kirch <okir@suse.de>
server: make Container.getChildByName() support the class testcase in addition to file

Signed-off-by: Olaf Kirch <okir@suse.de>
ni_testbus_client_container_child_by_name: do not print NameUnknown error; this may be intentional

Signed-off-by: Olaf Kirch <okir@suse.de>
testbus-client: support "delete --name" to delete test cases by their name, rather than their handle

Signed-off-by: Olaf Kirch <okir@suse.de>
selftest/functions: when creating a new top-level test group, flush out any existing left-overs from a previous run

Signed-off-by: Olaf Kirch <okir@suse.de>