test: use `T.Setenv` to set env vars in tests

This PR replaces os.Setenv with t.Setenv. Starting from Go 1.17, we can use t.Setenv to set environment variable in test. The environment variable is automatically restored to its original value when the test and all its subtests complete. This ensures that each test does not start with leftover environment variables from previous completed tests.

This saves us at least 2 lines (error check, and unsetting the env var) on every instance.

func TestFoo(t *testing.T) {
    // before
    key := "ENV"
    originalEnv := os.Getenv(key)

    if err := os.Setenv(key, "new value"); err != nil {
        t.Fatal(err)
    }
    defer func() {
        if err := os.Setenv(key, originalEnv); err != nil {
            t.Logf("failed to set env %s back to original value: %v", key, err)
        }
    }()

    // after
    t.Setenv(key, "new value")
}

Checklist

[x] Change is covered by automated tests
[x] The coding guidelines are followed
[ ] Public API has proper documentation in the Acra documentation site or has PR on documentation repository with new changes
[ ] CHANGELOG.md is updated (in case of notable or breaking changes)
[x] CHANGELOG_DEV.md is updated
[ ] Benchmark results are attached (if applicable)
are up-to-date (in case of API changes)

cossacklabs / acra

test: use `T.Setenv` to set env vars in tests #569

Checklist