Refactoring staking module, part 1

Introduced constant BONDED_DENOM for staking token denominator.
Introduced constant YEAR (only in tests), containing length of 1 year expressed in seconds.
Simplified arguments passed to setup_test_env helper function.
Unified and renamed variable test_env to env in all tests (shorter name).
Unified local variable names in all test cases.

NO FUNCTIONAL CHANGES

CosmWasm / cw-multi-test