Testing framework: support getting secret key from bootstrap accounts

type:added

For LIGO developers

Instead of using new_account / add_account, we can use bootstrap accounts and recover their secret keys.

Changelog details:

New Test function: get_bootstrap_account

The new function val get_bootstrap_account : nat -> address * key * string in Test retrieves the nth bootstrap account. Differently to nth_bootstrap_account, it also recovers the key and the secret key (represented as a string, similar to Test.new_account).