Avoid singleton random generators

[tovbinm]

Related issues
Random generators contain probabilityOfEmpty as their internal start. And since some of them were singletons, once probabilityOfEmpty was modified this would affect the values generated in other tests.

Describe the proposed solution
Avoid singleton generators instead always create a new random generator instance.

Describe alternatives you've considered
NA

[codecov]

Codecov Report

Merging #312 into master will not change coverage.
The diff coverage is 100%.

@@           Coverage Diff            @@
##           master     #312    +/-   ##
========================================
  Coverage   86.48%   86.48%            
========================================
  Files         322      322            
  Lines       10579    10579            
  Branches      343      564   +221     
========================================
  Hits         9149     9149            
  Misses       1430     1430

Impacted Files	Coverage Δ
...com/salesforce/op/testkit/ProbabilityOfEmpty.scala	100% <ø> (ø)	⬆️
...n/scala/com/salesforce/op/testkit/RandomText.scala	100% <100%> (ø)	⬆️

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[wsuchy]

OK