[Chore] Fix codecov report

[johnxnguyen]

What's in this PR?

Fixes: https://github.com/johnxnguyen/Down/issues/205

Issue

Codecov is reporting a much lower coverage than actually exists.

Cause

This appears to be to two main reasons:

1. Some directories are not being ignored, and
2. The coverage results from the multiple test runs aren't being merged.

Solution

1. Adjust the ignored paths
2. Upload coverage results after each test run

Also, to keep things simple, I've decided to move the snapshot tests into the main test test target and run them only for iOS.

[codecov[bot]]

Codecov Report

Merging #251 (7d7de50) into master (3ac921e) will increase coverage by 52.90%. The diff coverage is n/a.

@@             Coverage Diff             @@
##           master     #251       +/-   ##
===========================================
+ Coverage   38.41%   91.32%   +52.90%     
===========================================
  Files          80       60       -20     
  Lines        2262     1037     -1225     
===========================================
+ Hits          869      947       +78     
+ Misses       1393       90     -1303     

Impacted Files	Coverage Δ
Tests/DownTests/StringTests.swift
Tests/DownTests/BindingTests.swift
Tests/DownTests/AST/NodeTests.swift
Tests/DownTests/DownViewTests.swift
...nTests/Styler/Helpers/CGPoint_TranslateTests.swift
...DownTests/Styler/Helpers/CGRect_HelpersTests.swift
...yler/Helpers/NSAttributedString+HelpersTests.swift
Tests/DownTests/NSAttributedStringTests.swift
Tests/DownTests/AST/VisitorTests.swift
...rs/NSMutableAttributedString+AttributesTests.swift
... and 36 more

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