[BUG] Fix num input partitions in coalesce.

[clarkzinzow]

This PR fixes the number of input partitions in our Coalesce implementation. The new query planner was incorrectly using the current logical node's num_partitions for the number of input partitions, which for a Coalesce op, is equal to the output number of partitions; we should instead be using the input (child) logical node's num_partitions.

The microbenchmarking failure also illustrated that we didn't have any unit testing coverage of Coalesce (i.e. df.into_partitions(n) where n is smaller than the current number of partitions), so we also add a sanity unit test.

Closes #1434

[codecov]

Codecov Report

Merging #1442 (a88976e) into main (ecb596a) will increase coverage by 0.09%.
Report is 1 commits behind head on main.
The diff coverage is n/a.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #1442      +/-   ##
==========================================
+ Coverage   74.51%   74.61%   +0.09%     
==========================================
  Files          60       60              
  Lines        6035     6035              
==========================================
+ Hits         4497     4503       +6     
+ Misses       1538     1532       -6     

see 2 files with indirect coverage changes