apex test pattern for expecting a test.

gistfile1.txt

boolean codeDidThrowException = false;
string message;

try {
  code_that_will_cause_an_exception
} catch (System.calloutException sce) {
  codeDidThrowException = true;
  message = sce.getMessage();
}

system.assert(codeDidThrowException);
system.assertEquals('expected string', message);