Fix flaky tests

[giortzisg]

Description

This aims to fix flakiness of TestHTTPTransportDoesntLeakGoroutines. The test would report goroutines stuck in network I/O operations, where the http.DefaultClient would hang and then the goleak.VerifyNone would incorrectly spot them as leaked goroutines on the transport. The solution is adding a mockTransport to each test client we create on the tests.

In addition also added a small fix that might also cause goroutine issues. When using the time.After func, a goroutine would be spawned. Using ctx.Done() eliminates the goroutine and also improves readability.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 86.79%. Comparing base (6fa7ff4) to head (5383f4b).
⚠️ Report is 1 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #1071      +/-   ##
==========================================
- Coverage   87.08%   86.79%   -0.29%     
==========================================
  Files          55       55              
  Lines        5971     5962       -9     
==========================================
- Hits         5200     5175      -25     
- Misses        630      643      +13     
- Partials      141      144       +3     

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