make oasis-net-runner default fixtures more useful

[matevz]

This PR makes oasis-net-runner defaults more tailored to local (dApp) development and debugging:

• increases balance of test account to 10^15 base units (1 million TEST)
• sets max_allowances to 16 in order to make deposits to runtimes possible without passing a separate staking genesis fixture

Related oasis-sdk PR: https://github.com/oasisprotocol/oasis-sdk/pull/1081