build: npm install before running sample tests and avoid symlink in tests

[elierotenberg]

• Always npm install samples/package.json before running sample tests
• Copy, rather than symlink, in test.samples.auth

Partially fixes #2377.

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Fixes #2377 🦕

[codecov]

Codecov Report

Merging #2378 into master will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master    #2378   +/-   ##
=======================================
  Coverage   85.35%   85.35%           
=======================================
  Files          11       11           
  Lines        1714     1714           
  Branches       99       99           
=======================================
  Hits         1463     1463           
  Misses        251      251           

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