[Heartbeat] Fix flaky SSL test with retries. (elastic#10789)

Sometimes the httptest package when using fancy TLS options doesn't put the server up as fast as it should (at least that's the theory), and we hit before it's ready, causing a false test failure. This patch makes those tests more resilient.

It's possible there's something else at work here, but this bug is only seen on CI, and impossible to repro on my laptop.

Fixes elastic#10722

heartbeat/monitors/active/http/http_test.go

@@ -27,7 +27,9 @@ import (
 	"net/url"
 	"os"
 	"path"
+	"reflect"
 	"testing"
+	"time"
 
 	"github.com/stretchr/testify/require"
 
@@ -279,7 +281,16 @@ func runHTTPSServerCheck(
 		mergedExtraConfig[k] = v
 	}
 
-	event := testTLSRequest(t, server.URL, mergedExtraConfig)
+	// Sometimes the test server can take a while to start. Since we're only using this to test up statuses,
+	// we give it a few attempts to see if the server can come up before we run the real assertions.
+	var event *beat.Event
+	for i := 0; i < 10; i++ {
+		event = testTLSRequest(t, server.URL, mergedExtraConfig)
+		if v, err := event.GetValue("monitor.status"); err == nil && reflect.DeepEqual(v, "up") {
+			break
+		}
+		time.Sleep(time.Millisecond * 500)
+	}
 
 	mapval.Test(
 		t,