[Test] `Test.Blockchain` struct reuses one single backend implementation

[m-Peter]

Problem

Currently, when the Test.newEmulatorBlockchain() is called, it returns a new Test.Blockchain struct. However, the underlying backend implementation is the same for all Test.Blockchain structs that are created in a single test file. This is due to the fact that the ContractValueHandler injects only once the backend implementation, when it first encounters the Test contract in a test file.

Steps to Reproduce

import Test

pub let blockchain = Test.newEmulatorBlockchain()
pub let account = blockchain.createAccount()

pub fun testEventRetrieval() {
    let evts = blockchain.events()
    Test.assert(evts.length == 19)
    
    let blockchain2 = Test.newEmulatorBlockchain()
    Test.assert(blockchain2.events().length == 19)
}

Acceptance Criteria

Either returns a newly-created backend implementation for every call of Test.newEmulatorBlockchain(), or maybe move towards making the Test.Blockchain a singleton. Creating many backend implementations should add some overhead when running tests, and right now, it is possible to call Reset() on a Test.Blockchain value. See discussion in #138.

Context

onflow/developer-grants#148