Improve the use of goleak in unit tests

[vbanos]

Instead of using goleak in every unit test, run it in TestMain and it applies to all unit tests.

The project didn't have a TestMain so I created a main_test.go for that reason. TestMain could be added to any test file and it would work. I thought that it would be clearer to make a separate main_test.go file.

This method is also referenced in https://github.com/uber-go/goleak?tab=readme-ov-file#quick-start

[vbanos]

To verify this, I add the following leak in various places and it was caught by goleak in TestMain

go func() {
    time.Sleep(10 * time.Second) // Simulated leak: Goroutine never exits
}()

[CorentinB]

LGTM! Thanks @vbanos