Add random test feature generator to generate datasets with features of *all* types

[tovbinm]

Related issues
TestFeatureBuilder can only generate test features & dataframe for upto 5 features, but we have 51 different types total! We should be able to test all of them easily.

Describe the proposed solution

1. Add random test feature generator - TestFeatureBuilder.random that allows generating all feature types
2. Add test feature generator for arbitrary number of features
3. I had to move some files around
4. Removed some unused code

[codecov]

Codecov Report

Merging #298 into master will increase coverage by 0.06%.
The diff coverage is 96.96%.

@@            Coverage Diff             @@
##           master     #298      +/-   ##
==========================================
+ Coverage   86.35%   86.42%   +0.06%     
==========================================
  Files         321      322       +1     
  Lines       10539    10570      +31     
  Branches      354      572     +218     
==========================================
+ Hits         9101     9135      +34     
+ Misses       1438     1435       -3

Impacted Files	Coverage Δ
...n/scala/com/salesforce/op/testkit/RandomData.scala	100% <ø> (ø)	⬆️
...scala/com/salesforce/op/test/FeatureTestBase.scala	0% <ø> (ø)
...n/scala/com/salesforce/op/testkit/RandomText.scala	100% <ø> (ø)	⬆️
...n/scala/com/salesforce/op/testkit/RandomReal.scala	90.47% <100%> (ø)	⬆️
...la/com/salesforce/op/test/TestFeatureBuilder.scala	100% <100%> (ø)
.../scala/com/salesforce/op/test/FeatureAsserts.scala	100% <100%> (ø)
...scala/com/salesforce/op/testkit/RandomVector.scala	90.47% <92.3%> (+4.76%)	⬆️

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update 213962e...831c31d. Read the comment docs.