[#2593] Fix missing coverage data in codecov

[swrichards]

Previously our coverage data was imporperly being shipped to codecov on
two accounts:

1. We were not combining the data files from each run of the test matrix
   (elastic and main)
2. The main test run was not running with the appropriate --concurrency
   flag and thus it was missing out on coverage data due to not taking
   into account all test processes.

This commit addresses both issues and ensures codecov receives the
full coverage data file (both runs, all parallel processes).

[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 95.13%. Comparing base (abc196e) to head (22a1ad6).

Additional details and impacted files

@@             Coverage Diff              @@
##           develop    #1290       +/-   ##
============================================
+ Coverage    44.23%   95.13%   +50.90%     
============================================
  Files          973      983       +10     
  Lines        35625    35837      +212     
============================================
+ Hits         15757    34094    +18337     
+ Misses       19868     1743    -18125     

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