Tests: use t.Setenv to set env vars (#69516)

This commit replaces `os.Setenv` with `t.Setenv` in tests. The
environment variable is automatically restored to its original value
when the test and all its subtests complete.

Reference: https://pkg.go.dev/testing#T.Setenv

Signed-off-by: Eng Zer Jun <engzerjun@gmail.com>

pkg/setting/dynamic_settings_test.go

@@ -1,7 +1,6 @@
 package setting
 
 import (
-	"os"
 	"testing"
 
 	"github.com/stretchr/testify/require"
@@ -14,12 +13,7 @@ func TestDynamicSettingsSupport_Override(t *testing.T) {
 	keyName := "bar"
 	expected := "dynamic value"
 
-	err := os.Setenv(envKey, expected)
-	require.NoError(t, err)
-	defer func() {
-		err := os.Unsetenv(envKey)
-		require.NoError(t, err)
-	}()
+	t.Setenv(envKey, expected)
 
 	value := cfg.SectionWithEnvOverrides(sectionName).Key(keyName).MustString("default value")
 	require.Equal(t, expected, value)