mw/health: poll other middleware (#976)

This add the infrastructure to let other middleware report their health
status back to the health middleware. A health.Healther interface is
introduced and a middleware needs to implement that. A middleware
that supports healthchecks is statically configured.

Every second each supported middleware is queried and the global health
state is updated.

Actual tests have been disabled as no other middleware implements this
at the moment.

middleware/health/setup_test.go

@@ -0,0 +1,35 @@
+package health
+
+import (
+	"testing"
+
+	"github.com/mholt/caddy"
+)
+
+func TestSetupHealth(t *testing.T) {
+	tests := []struct {
+		input     string
+		shouldErr bool
+	}{
+		{`health`, false},
+		{`health localhost:1234`, false},
+		{`health bla:a`, false},
+		{`health bla`, true},
+		{`health bla bla`, true},
+	}
+
+	for i, test := range tests {
+		c := caddy.NewTestController("dns", test.input)
+		_, err := healthParse(c)
+
+		if test.shouldErr && err == nil {
+			t.Errorf("Test %d: Expected error but found %s for input %s", i, err, test.input)
+		}
+
+		if err != nil {
+			if !test.shouldErr {
+				t.Errorf("Test %d: Expected no error but found one for input %s. Error was: %v", i, test.input, err)
+			}
+		}
+	}
+}