Its best to just do one atomic "test" per method in this class. I'd probably break this up into 

 - `testAssignVariable`
 - `testAssignVariableMultipleAssignment`
 - `testAssignVariableNotDefined`

Also note that junit has some built-in exception checkers http://stackoverflow.com/questions/156503/how-do-you-assert-that-a-certain-exception-is-thrown-in-junit-4-tests