test(s3 remote cache): wait for the master to list the filer before shell commands (#12018)

A fresh single-filer mini registers its filer only after the ~10s
empty-membership confirm window, and weed shell locates the filer via
the master's listing, so remote.configure sent right after the S3
health check races it. Ports the wait from the enterprise test to the
OSS copy.

test/s3/remote_cache/remote_cache_evict_test.go

@@ -99,6 +99,27 @@ func shellOn(t *testing.T, masterPort, command string) string {
 	return stripLogs(string(out))
 }
 
+// waitForFilerListed polls the master until it lists a filer. A fresh
+// single-filer mini registers its filer only after an ~10 s empty-membership
+// confirm, and weed shell finds the filer through that listing, so shell
+// commands sent before it fail.
+func waitForFilerListed(t *testing.T, masterPort string) {
+	t.Helper()
+	deadline := time.Now().Add(60 * time.Second)
+	for {
+		cmd := exec.Command(weedBinary, "shell", "-master=localhost:"+masterPort)
+		cmd.Stdin = strings.NewReader("cluster.ps\nexit\n")
+		out, err := cmd.CombinedOutput()
+		if err == nil && strings.Contains(string(out), "* filers ") && !strings.Contains(string(out), "* filers 0\n") {
+			return
+		}
+		if time.Now().After(deadline) {
+			t.Fatalf("master on %s lists no filer after 60s: %v\n%s", masterPort, err, out)
+		}
+		time.Sleep(time.Second)
+	}
+}
+
 func chunkCountOn(t *testing.T, masterPort, path string) string {
 	meta := shellOn(t, masterPort, "fs.meta.cat "+path)
 	idx := strings.LastIndex(meta, "chunks ")
@@ -166,6 +187,7 @@ func TestRemoteCacheEvictUnderPressure(t *testing.T) {
 		"-master.volumeSizeLimitMB=32", "-volume.max=2",
 		"-filer.remoteCacheEvictThreshold=0.99")
 	waitForHTTP(t, evictPrimaryS3)
+	waitForFilerListed(t, evictPrimaryMaster)
 
 	remote := createS3Client(evictRemoteS3)
 	_, err := remote.CreateBucket(&s3.CreateBucketInput{Bucket: aws.String(evictBucket)})