Avoid assertion in finally block

[exoego]

This is an issue found while working in #232.

Assertion in finally block is generally a not good idea.
Because it hides the real exception thrown in try block and makes debugging hard.

try {
  doSomething();   // unexpected exception thrown
  assertThat(...);
} finally {
  assertThat(...); // AssertionError is thrown, which hides the exception thrown from try block
}