How to exclude file from PHPUnit test suite in xml config?

To exclude the file name TestCase.php.

add this to your phpunit.xml

<testsuites>
    <testsuite name="BLABLA">
        <directory suffix=".php">./tests</directory>
        <exclude>./tests/TestCase.php</exclude>
    </testsuite>
</testsuites>

Here is an additional excerpt from a real-live test-suite I can confirm it working with:

...
    <testsuites>
        <testsuite name="n98-magerun-tests">
            <directory>./tests</directory>
            <exclude>tests/N98/Magento/Command/Installer/UninstallCommandTest.php</exclude>
        </testsuite>
    ...