Hi,
There are a couple of options for the phpunit command that can help define which tests should and should not be run :
$ phpunit --help
PHPUnit 3.4.0beta5 by Sebastian Bergmann.
Usage: phpunit [switches] UnitTest [UnitTest.php]
       phpunit [switches] <directory>
...
  --filter <pattern>       Filter which tests to run.
  --group ...              Only runs tests from the specified group(s).
  --exclude-group ...      Exclude tests from the specified group(s).
  --list-groups            List available test groups.
...
Those will probably not allow to specify exactly what you wanted... But they might help.
For more details, see The Command-Line Test Runner
Especially, I kind of like the group feature : just use a @group tag in the phpdoc of your tests, to re-group them (for instance, one group per "chunk of functionnality") ; and, after, you can use the --group or --exclude-group options on the command line to specify which tests should or shouldn't be run.
In your case, if you cannot modify the tests, maybe the --filter option can help, depending on how your tests are nammed (ie, if there is a way to identify those you want to run) :
  --filter
  
Only runs tests whose name matches the given pattern. The pattern can be
  either the name of a single test or a
  regular expression that matches
  multiple test names.
Another solution, if you are not always changing the "groups" of tests to run, might be to use a test-suite that only includes the tests you want to run.
For instance, take a look at Composing a Test Suite Using XML Configuration.