Allow has_equal_value-like behavior from pythonwhat

[filipsch]

Currently, in if tests, for loops and function definitions, it is not possible to rerun the piece of code that is zoomed in on like it is possible in pythonwhat with has_equal_value(), has_equal_error() and has_equal_output(). It shouldn't be that hard to include this functionality as well. What it would look like:

# check if condition of an if-else evaluates correctly
ex() %>% check_if_else() %>% {
  check_cond(.) %>% check_expr() %>% check_result() %>% check_equal()
  check_if() %>% ...
  check_else() %>% ...
}

# check if value of a variable total is properly updated in the body (where to put "total"?)
ex() %>% check_for_loop() %>% {
  check_cond(.) %>% ...
  check_body(.) %>% check_expr() %>% check_result("total") %>% check_equal("total")
}

# todo add more ...

cc @sumedh10 if you have any ideas on what a good API would be, let me know.