I agree, testing them out of container in your situation will get you quite far. If you do have the need to run them in-container, take a look at
TagUnit. It's an in-container testing framework specifically for testing custom tags and in addition to just helping you test the tags, it looks at potential problems in the TLD file. For example, those times when you've forgotten to add a setter method for an attribute or missed the tag handler class from your webapp's classpath.
As with all testing, you'll probably need to use multiple tools to get a good coverage and overall confidence.