Add EmulatorBackend.Reset method and beforeEach / afterEach hooks

[m-Peter]

Work towards: onflow/developer-grants#148

Description

The general idea here, is to have better control at resetting the state of the blockchain, and some flexible hooks to run before/after each test case. For example:

import Test

pub let blockchain = Test.newEmulatorBlockchain()

pub fun beforeEach() {
    // Reset the state of the blockchain
    blockchain.reset()
}

// or the afterEach() can be used to reset the state
pub fun afterEach() {
    blockchain.reset()
}

This feature was first discussed about in https://github.com/orgs/onflow/discussions/1330

---

• Targeted PR against master branch
• Linked to Github issue with discussion and accepted design OR link to spec that describes this work
• Code follows the standards mentioned here
• Updated relevant documentation
• Re-reviewed Files changed in the Github PR explorer
• Added appropriate labels

[turbolent]

Nice! 👌