Rework syntax for creating updates in tests

[fthomas]

This reworks the syntax for creating updates in tests. It

• does not use the Update.Single.apply and Update.Group.apply methods anymore. This will make it easier to change both classes since we need to update the apply method in a lot less places.
• gets rid of implicit conversions in TestSyntax. The syntax is now based on extension methods via implicit classes only.
• makes creating Updates more concise.

[codecov]

Codecov Report

Merging #2377 (12783e6) into master (fd909aa) will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master    #2377   +/-   ##
=======================================
  Coverage   80.42%   80.42%           
=======================================
  Files         139      139           
  Lines        2391     2391           
  Branches       51       51           
=======================================
  Hits         1923     1923           
  Misses        468      468           

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[mzuehlke]

That's easier to read 👍