mount: enable debug logging for the flaky TestMount test

The test case fails from time to time with an Input/Output error while
trying to access the snapshots directory.

internal/debug/testing.go

@@ -0,0 +1,23 @@
+package debug
+
+import (
+	"log"
+	"os"
+	"testing"
+)
+
+// TestLogToStderr configures debug to log to stderr if not the debug log is
+// not already configured and returns whether logging was enabled.
+func TestLogToStderr(t testing.TB) bool {
+	if opts.isEnabled {
+		return false
+	}
+	opts.logger = log.New(os.Stderr, "", log.LstdFlags)
+	opts.isEnabled = true
+	return true
+}
+
+func TestDisableLog(t testing.TB) {
+	opts.logger = nil
+	opts.isEnabled = false
+}