Fix/Optimize test termination condition check (esnet#1114)

`test->done` represents the test completion.
In some modes, duration-based (-t) test, and file-transfer (-F)  in particular,
the `test->done` is set during the timeout handler or file-transfer functions.
For such configurations, and in general, `test->done` being set is sufficient
condition for terminating the test.

This commit generalizes the condition check to `test->done`,
removing the clause which limtis this condition case to duration-based test only.

src/iperf_client_api.c

@@ -555,7 +555,7 @@ iperf_run_client(struct iperf_test * test)
 	     * being the receiver.
 	     */
 	    if ((!test->omitting) &&
-	        ((test->duration != 0 && test->done) ||
+	        (test->done ||
 	         (test->settings->bytes != 0 && (test->bytes_sent >= test->settings->bytes ||
 						 test->bytes_received >= test->settings->bytes)) ||
 	         (test->settings->blocks != 0 && (test->blocks_sent >= test->settings->blocks ||