I found an unexpected use of assertion in the block of `assert_raise`
when I implemented https://github.com/rubocop/rubocop-minitest/pull/137.
It is expected to be asserted after an exception is raised in
`assert_raise` block, but in actually it is not asserted after an
exception is raised. Therefore, this PR removes or updates assertions
that have not been asserted after an exception has raised.

This PR will add `rubocop-minitest` and enable
`Minitest/UnreachableAssertion` cop to able similar auto-detection,
but will remove `rubocop-minitest` from this PR if you don't like it.