A custom test driver also has the task of displaying, on the standard output, the test results as soon as they become available. Depending on the protocol in use, it can also display the reasons for failures and skips, and, more generally, any useful diagnostic output (but remember that each line on the screen is precious, so that cluttering the screen with overly verbose information is bad idea). The exact format of this progress output is left up to the test driver; in fact, a custom test driver might theoretically even decide not to do any such report, leaving it all to the testsuite summary (that would be a very lousy idea, of course, and serves only to illustrate the flexibility that is granted here).
Remember that consistency is good; so, if possible, try to be consistent with the output of the built-in Automake test drivers, providing a similar “look & feel”. In particular, the testsuite progress output should be colorized when the --color-tests is passed to the driver. On the other end, if you are using a known and widespread test protocol with well-established implementations, being consistent with those implementations' output might be a good idea too.