The test case is not recognized by the Simpletest module
As srihita pointed out in a comment, you need to add the file which contains the test class to the files[] array in your module's info file like this:

files[]=yourmodule.test


Then, let it rebuild by submitting the modules page or use the "Clear information" (or similar, not sure about the exact label right now) at the bottom of the Testing page.

Also, you're code is missing a ', but that might be a copy/paste error.