Fail tests if unhandled panics happened.

We normally recover from those situations, printing stack traces instead of
crashing the program. But during tests, we're not looking at the prometheus
metrics or all the output. Without these checks, such panics could go
unnoticed. Seems like a reasonable thing to add, unhandled panics haven't been
encountered in tests.

dmarcdb/main_test.go

@@ -0,0 +1,17 @@
+package dmarcdb
+
+import (
+	"fmt"
+	"os"
+	"testing"
+
+	"github.com/mjl-/mox/metrics"
+)
+
+func TestMain(m *testing.M) {
+	m.Run()
+	if metrics.Panics.Load() > 0 {
+		fmt.Println("unhandled panics encountered")
+		os.Exit(2)
+	}
+}