Don't report unused private properties in expect class

[schalkms]

This is a subsequent commit of #2643.
Further tests are added that check unused elements in expect and actual classes.

[codecov]

Codecov Report

Merging #2646 into master will decrease coverage by 0.01%.
The diff coverage is 0.00%.

@@             Coverage Diff              @@
##             master    #2646      +/-   ##
============================================
- Coverage     80.01%   80.00%   -0.02%     
  Complexity     2292     2292              
============================================
  Files           379      379              
  Lines          6761     6761              
  Branches       1222     1222              
============================================
- Hits           5410     5409       -1     
  Misses          740      740              
- Partials        611      612       +1     

Impacted Files	Coverage Δ	Complexity Δ
...turbosch/detekt/rules/style/UnusedPrivateMember.kt	91.26% <0.00%> (-0.98%)	5.00 <0.00> (ø)

---

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 6e8e5ae...9821ad9. Read the comment docs.