[receiver/prometheusremotewrite] flaky test (#37563)

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Ex. Adding a feature - Explain what this achieves.-->
#### Description

Using httptest package instead of starting a real server avoiding port
conflicts.

#### Link to tracking issue
Fixes
#36654

receiver/prometheusremotewritereceiver/receiver_test.go

@@ -8,6 +8,7 @@ import (
 	"context"
 	"fmt"
 	"net/http"
+	"net/http/httptest"
 	"testing"
 
 	"github.com/gogo/protobuf/proto"
@@ -16,7 +17,6 @@ import (
 	writev2 "github.com/prometheus/prometheus/prompb/io/prometheus/write/v2"
 	"github.com/prometheus/prometheus/storage/remote"
 	"github.com/stretchr/testify/assert"
-	"go.opentelemetry.io/collector/component/componenttest"
 	"go.opentelemetry.io/collector/consumer/consumertest"
 	"go.opentelemetry.io/collector/pdata/pmetric"
 	"go.opentelemetry.io/collector/receiver/receivertest"
@@ -58,22 +58,7 @@ func setupMetricsReceiver(t *testing.T) *prometheusRemoteWriteReceiver {
 	return prwReceiver.(*prometheusRemoteWriteReceiver)
 }
 
-func setupServer(t *testing.T) {
-	t.Helper()
-
-	prwReceiver := setupMetricsReceiver(t)
-	ctx, cancel := context.WithCancel(context.Background())
-	t.Cleanup(cancel)
-
-	assert.NoError(t, prwReceiver.Start(ctx, componenttest.NewNopHost()))
-	t.Cleanup(func() {
-		assert.NoError(t, prwReceiver.Shutdown(ctx), "Must not error shutting down")
-	})
-}
-
 func TestHandlePRWContentTypeNegotiation(t *testing.T) {
-	setupServer(t)
-
 	for _, tc := range []struct {
 		name         string
 		contentType  string
@@ -113,13 +98,16 @@ func TestHandlePRWContentTypeNegotiation(t *testing.T) {
 
 			var compressedBody []byte
 			snappy.Encode(compressedBody, pBuf.Bytes())
-			req, err := http.NewRequest(http.MethodPost, "http://localhost:9090/api/v1/write", bytes.NewBuffer(compressedBody))
-			assert.NoError(t, err)
+
+			req := httptest.NewRequest(http.MethodPost, "/api/v1/write", bytes.NewBuffer(compressedBody))
 
 			req.Header.Set("Content-Type", tc.contentType)
 			req.Header.Set("Content-Encoding", "snappy")
-			resp, err := http.DefaultClient.Do(req)
-			assert.NoError(t, err)
+			w := httptest.NewRecorder()
+
+			prwReceiver := setupMetricsReceiver(t)
+			prwReceiver.handlePRW(w, req)
+			resp := w.Result()
 
 			assert.Equal(t, tc.expectedCode, resp.StatusCode)
 			if tc.expectedCode == http.StatusNoContent { // We went until the end