Using a form of "ST-operational semantics" we develop a noninterleaving semantic theory of processes based on testing. This operational semantics is based on the assumption that all actions have a non-zero duration and the allowed tests can therefore distinguish between the beginning and the termination of actions. The result is a semantic theory in which concurrency is differentiated from nondeterminism. We show that the semantic preorder based on these tests is preserved by so-called "stable" action refinements and may be characterised as the largest such preorder contained in the standard testing preorder.