Ensure teardown is always executed

[chitoku-k]

In some cases especially when tests failed due to err_exit, the teardown function was not executed. This PR uses trap functions to ensure it is always executed.

@setup {
  tempfile=test
  touch $tempfile
}

@teardown {
  # Here is not executed in some of the following cases
  rm $tempfile
}

@test 'case ok' {
  # This case is totally OK because `run` temporarily disables `err_exit` (using unsetopt)
  run false
}

@test 'case ng' {
  # This PR fixes this case where the test fails with `err_exit` being set
  false
}

I tried to show this by adding test cases that cover them which resulted in the entire test to break because it needs to be somehow failed...