Allow moveTime function to accept negative time deltas for going backwards in time

[m-Peter]

Companion PR: onflow/cadence-tools#196

Description

Change the delta argument type from UFix64 to Fix64.

import Test

pub fun testMoveBackward() {
    let blockchain = Test.newEmulatorBlockchain()
    // timeDelta is the representation of 35 days,
    // in the form of seconds. A negative time delta
    // signifies going back in time.
    let timeDelta = Fix64(35 * 24 * 60 * 60) * -1.0
    blockchain.moveTime(by: timeDelta)
}

---

• 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

Merging #2734 (a8e229a) into master (893aea8) will not change coverage.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##           master    #2734   +/-   ##
=======================================
  Coverage   79.15%   79.15%           
=======================================
  Files         333      333           
  Lines       78379    78379           
=======================================
  Hits        62037    62037           
  Misses      14044    14044           
  Partials     2298     2298           

Flag	Coverage Δ
unittests	79.15% <100.00%> (ø)

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

Files Changed	Coverage Δ
runtime/stdlib/test_emulatorbackend.go	54.94% <100.00%> (ø)