Add utility methods to EE testing framework.

We have tons of boilerplate around creating accounts, funding, bonding and unbonding. Each of these operations requires 5+ lines of code. This makes it very tedious to write the code and bloats tests to an extend where setting up a test scenario can take over 100 lines of code.
Add utility methods that will:
transfer tokens
create accounts
(un)bond
Refactor current tests to use those methods.
Sample: https://github.com/casper-network/casper-node/pull/2616#discussion_r814050150