Configure code coverage reporting

[jsbean]

This PR refines the configuration of the code coverage gathering process in several ways:

• Ask codecov.io to ignore the Tests directory for a more honest view of coverage
• Only generate-xcodeproj if we are on a Mac
• Only gather coverage if we are on a Mac

[codecov-io]

Codecov Report

Merging #77 into latest will decrease coverage by 48.97%.
The diff coverage is n/a.

@@             Coverage Diff             @@
##           latest      #77       +/-   ##
===========================================
- Coverage   85.06%   36.08%   -48.98%     
===========================================
  Files         129       43       -86     
  Lines       20734     1283    -19451     
===========================================
- Hits        17637      463    -17174     
+ Misses       3097      820     -2277

Impacted Files	Coverage Δ
...dTests/Timewise/03_Rhythm/Timewise_03_Rhythm.swift
Tests/MusicXMLTests/Complex Types/RestTests.swift
.../Partwise/12_Clefs/Partwise_12_Clefs_A_Clefs.swift
...cXMLTests/Complex Types/PartwiseMeasureTests.swift
Tests/MusicXMLTests/HelloWorld.swift
...ondTests/Timewise/02_Rests/Timewise_02_Rests.swift
...mewise_01_Pitches_E_ParenthesizedAccidentals.swift
...ts/Timewise/23_Tuplets/Timewise_23_Tuplets_A.swift
Tests/MusicXMLTests/Complex Types/KeyTests.swift
..._Pitches_F_ParenthesizedMicrotoneAccidentals.swift
... and 76 more

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