tests/npm-adapter: Use async/await instead of promise chains

[Turbo87]

This should make the test code a little easier on the eyes :)

[codecov]

Codecov Report

Merging #788 (01179f5) into master (cb5f901) will decrease coverage by 1.01%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master     #788      +/-   ##
==========================================
- Coverage   95.46%   94.44%   -1.02%     
==========================================
  Files          16       17       +1     
  Lines         463      558      +95     
==========================================
+ Hits          442      527      +85     
- Misses         21       31      +10     

Impacted Files	Coverage Δ
lib/utils/dependency-manager-adapter-factory.js	90.62% <0.00%> (-4.38%)	⬇️
lib/dependency-manager-adapters/pnpm.js	90.36% <0.00%> (ø)

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[kategengler]

Thank you!