Refactor test_vm::utils in terms of an abstract VM trait

[alexytsu]

Next step of #1236

Moves enough of the functionality of the current TestVM into the VM trait such that the all the utils can be factored in terms of the trait.

Some functionality has shifted to immutable patterns (e.g. with_epoch -> set_epoch). The trait assumes non-mutable borrows so mutability is achieved with RefCell when needed. The tests will be refactored in terms of the trait in a followup PR.

There's some duplication at the moment between concrete behaviour of TestVM and behaviours in the VM trait but these will be easier to remove in a follow-up (when the tests themselves are refactored in terms of the VM trait

[codecov-commenter]

Codecov Report

Merging #1269 (8fea3f2) into master (f393b91) will decrease coverage by 0.20%.
The diff coverage is 92.16%.

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #1269      +/-   ##
==========================================
- Coverage   90.88%   90.68%   -0.20%     
==========================================
  Files         133      133              
  Lines       26755    26880     +125     
==========================================
+ Hits        24316    24377      +61     
- Misses       2439     2503      +64     

Impacted Files	Coverage Δ
actors/power/src/lib.rs	83.01% <ø> (-0.58%)	⬇️
test_vm/src/util.rs	98.84% <91.33%> (-1.08%)	⬇️
test_vm/src/lib.rs	84.09% <93.33%> (+0.93%)	⬆️

... and 16 files with indirect coverage changes

[alexytsu]

Eventually will move TestVM to a separate submodule, but I've left it in this file so the diff is clear

[alexytsu]

This replaces with_epoch. However, all usages of with_epoch in the current tests were

let v = v.with_epoch(200);

so the non-mutability/cloneability wasn't being used