Use async FS in tests

[DeMoorJasper]

This PR refactors the tests to use async FS instead of sync FS, to be more inline with the code of parcel.

Also cleans up rimraf and ncp being copied and promisified in every test that used it.

This would also be usefull to port the tests to use virtual fs in the future for better test stability and performance (when using memfs), as the workers need to share the same fs as the master process, which can only occur asynchronously... (ipc)

[codecov-io]

Codecov Report

Merging #1437 into master will increase coverage by 3.89%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master    #1437      +/-   ##
==========================================
+ Coverage   86.81%   90.71%   +3.89%     
==========================================
  Files          80       80              
  Lines        4369     6029    +1660     
==========================================
+ Hits         3793     5469    +1676     
+ Misses        576      560      -16

Impacted Files	Coverage Δ
src/assets/WebManifestAsset.js	59.18% <0%> (-19.39%)	⬇️
src/Logger.js	83.06% <0%> (-12.73%)	⬇️
src/packagers/CSSPackager.js	87.87% <0%> (-12.13%)	⬇️
src/utils/getRootDir.js	86.11% <0%> (-8.34%)	⬇️
src/HMRServer.js	85% <0%> (-6.67%)	⬇️
src/Bundle.js	93.05% <0%> (-6.2%)	⬇️
src/packagers/JSPackager.js	82.74% <0%> (-5.83%)	⬇️
src/Pipeline.js	93.06% <0%> (-4.89%)	⬇️
src/utils/bundleReport.js	67.04% <0%> (-4.75%)	⬇️
src/FSCache.js	93.22% <0%> (-3.45%)	⬇️
... and 41 more

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