fix test coverage

[hannessolo]

There is a bug with c8 and esmock that breaks test coverage when mocking a global like:

await esmock('some-module.js', {}, {
  import: {
    fetch: mockFetch
  }
});

Instead, we should use:

async function withMockedFetch(act) {
  const savedFetch = globalThis.fetch;
  globalThis.fetch = fetch;
  try {
    await act();
  } finally {
    globalThis.fetch = savedFetch;
  }
}

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 86.71%. Comparing base (78a68cf) to head (4d2f753).
Report is 1 commits behind head on main.

Additional details and impacted files

@@             Coverage Diff             @@
##             main     #156       +/-   ##
===========================================
+ Coverage   76.23%   86.71%   +10.47%     
===========================================
  Files          39       39               
  Lines        2243     2243               
===========================================
+ Hits         1710     1945      +235     
+ Misses        533      298      -235     

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