Add functions on testing framework's blockchain to create/load snapshots

[m-Peter]

Description

Allow developers to create snapshots of the blockchain's current ledge state, and load them on demand at any time. For example:

import Test
import BlockchainHelpers

pub let blockchain = Test.newEmulatorBlockchain()
pub let helpers = BlockchainHelpers(blockchain: blockchain)

pub fun test() {
    let admin = blockchain.createAccount()
    blockchain.createSnapshot("adminCreated")

    helpers.mintFlow(to: admin, amount: 1000.0)
    blockchain.createSnapshot("adminFunded")

    var balance = helpers.getFlowBalance(for: admin)
    Test.assertEqual(1000.0, balance)

    blockchain.loadSnapshot("adminCreated")
            
    balance = helpers.getFlowBalance(for: admin)
    Test.assertEqual(0.0, balance)

    blockchain.loadSnapshot("adminFunded")
            
    balance = helpers.getFlowBalance(for: admin)
    Test.assertEqual(1000.0, balance)
}

---

• 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

[codecov]

Codecov Report

Patch coverage: 92.00% and project coverage change: +0.03% 🎉

Comparison is base (5254952) 79.18% compared to head (0c10225) 79.21%.

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #2763      +/-   ##
==========================================
+ Coverage   79.18%   79.21%   +0.03%     
==========================================
  Files         333      333              
  Lines       78614    78664      +50     
==========================================
+ Hits        62248    62313      +65     
+ Misses      14063    14044      -19     
- Partials     2303     2307       +4     

Flag	Coverage Δ
unittests	79.21% <92.00%> (+0.03%)	⬆️

Flags with carried forward coverage won't be shown. Click here to find out more.

Files Changed	Coverage Δ
runtime/stdlib/test_emulatorbackend.go	58.27% <92.00%> (+3.33%)	⬆️

... and 1 file with indirect coverage changes

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[m-Peter]

Credits to @bluesign for his original idea 👏

[turbolent]

Awesome!

[bluesign]

this is awesome!

[turbolent]

Nice!