test: Fix flaky TestServer/Logging/{Multiple,Stackdriver} (#5727)

* test: Fix flaky TestServer/Logging/Multiple

* test: Fix flaky TestServer/Logging/Stackdriver

* test: Add testutil.TempFile and testutil.CreateTemp, cleanup tests
        relying on temp file

testutil/temp.go

@@ -0,0 +1,47 @@
+package testutil
+
+import (
+	"os"
+	"testing"
+
+	"github.com/stretchr/testify/require"
+)
+
+// TempFile returns the name of a temporary file that does not exist.
+func TempFile(t *testing.T, dir, pattern string) string {
+	t.Helper()
+
+	if dir == "" {
+		dir = t.TempDir()
+	}
+	f, err := os.CreateTemp(dir, pattern)
+	require.NoError(t, err, "create temp file")
+	name := f.Name()
+	err = f.Close()
+	require.NoError(t, err, "close temp file")
+	err = os.Remove(name)
+	require.NoError(t, err, "remove temp file")
+
+	return name
+}
+
+// CreateTemp is a convenience function for creating a temporary file, like
+// os.CreateTemp, but it also registers a cleanup function to close and remove
+// the file.
+func CreateTemp(t *testing.T, dir, pattern string) *os.File {
+	t.Helper()
+
+	if dir == "" {
+		dir = t.TempDir()
+	}
+	f, err := os.CreateTemp(dir, pattern)
+	require.NoError(t, err, "create temp file")
+	t.Cleanup(func() {
+		_ = f.Close()
+		err = os.Remove(f.Name())
+		if err != nil {
+			t.Logf("CreateTemp: Cleanup: remove failed for %q: %v", f.Name(), err)
+		}
+	})
+	return f
+}