housekeeping: Added IBuilder and Extensions

[RLittlesII]

• Moved extensions to correctly named files

What kind of change does this PR introduce?

Adds IBuilder and extensions for building objects.

What is the current behavior?

This currently isn't part of ReactiveUI.Testing

What is the new behavior?

You can now use the IBuilder and extensions by sourcing ReactiveUI.Testing

What might this PR break?

Nothing.

Please check if the PR fulfills these requirements

• Tests for the changes have been added (for bug fixes / features)
• Docs have been added / updated (for bug fixes / features)

Other information:

[codecov]

Codecov Report

Merging #2019 into master will decrease coverage by 0.22%.
The diff coverage is 0%.

@@            Coverage Diff             @@
##           master    #2019      +/-   ##
==========================================
- Coverage      61%   60.78%   -0.23%     
==========================================
  Files         113      115       +2     
  Lines        4565     4580      +15     
  Branches      655      656       +1     
==========================================
- Hits         2785     2784       -1     
- Misses       1598     1614      +16     
  Partials      182      182

Impacted Files	Coverage Δ
src/ReactiveUI.Testing/SchedulerExtensions.cs	58.69% <ø> (ø)
src/ReactiveUI.Testing/MessageBusExtensions.cs	0% <0%> (ø)
src/ReactiveUI.Testing/IBuilderExtensions.cs	0% <0%> (ø)

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