Update dependency Microsoft.Reactive.Testing to v6

[renovate]

This PR contains the following updates:

Package	Type	Update	Change
Microsoft.Reactive.Testing	nuget	major	5.0.0 -> 6.0.0

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Configuration

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[codecov]

Codecov Report

Merging #974 (e74e780) into master (d6157dc) will increase coverage by 2.64%.
Report is 19 commits behind head on master.
The diff coverage is n/a.

❗ Current head e74e780 differs from pull request most recent head 7112899. Consider uploading reports for the commit 7112899 to get more accurate results

@@            Coverage Diff             @@
##           master     #974      +/-   ##
==========================================
+ Coverage   69.72%   72.37%   +2.64%     
==========================================
  Files         493      255     -238     
  Lines       20066    12847    -7219     
  Branches     1603      817     -786     
==========================================
- Hits        13991     9298    -4693     
+ Misses       6075     3549    -2526     

see 242 files with indirect coverage changes