wdio-sync: convert executeAsync to async/await

[MatthewBurstein]

Proposed changes

Convert executeAsync in @wdio/sync to async/await.

Types of changes

• Bugfix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)
• Documentation update

Checklist

• I have read the CONTRIBUTING doc
• I have added tests that prove my fix is effective or that my feature works
• I have added necessary documentation (if appropriate)

Further comments

Reviewers: @webdriverio/technical-committee

[codecov]

Codecov Report

Merging #4620 into master will decrease coverage by <.01%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master    #4620      +/-   ##
==========================================
- Coverage   99.33%   99.33%   -0.01%     
==========================================
  Files         195      195              
  Lines        5117     5109       -8     
  Branches     1099     1096       -3     
==========================================
- Hits         5083     5075       -8     
  Misses         31       31              
  Partials        3        3

Impacted Files	Coverage Δ
packages/wdio-sync/src/index.js	100% <100%> (ø)	⬆️

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[christian-bromann]

👍

[colinbendell]

this is problematic for this.skip() calls which is a throw new Pending('sync skip'). This used to fall out of executeAsync() by being wrapped in a Promise.reject(). Now, it is caught and rethrown after prettying the e.stack. However, e.stack doesn't exist and so any use of this.skip() results in Cannot read property 'split' of undefined