s3tables: enhance test robustness and resilience

Updated random string generation to use crypto/rand in s3tables tests.
Increased resilience of IAM distributed tests by adding "connection refused"
to retryable errors.

test/s3/iam/s3_iam_distributed_test.go

@@ -129,6 +129,7 @@ func TestS3IAMDistributedTests(t *testing.T) {
 			errorMsg := err.Error()
 			return strings.Contains(errorMsg, "timeout") ||
 				strings.Contains(errorMsg, "connection reset") ||
+				strings.Contains(errorMsg, "connection refused") ||
 				strings.Contains(errorMsg, "temporary failure") ||
 				strings.Contains(errorMsg, "TooManyRequests") ||
 				strings.Contains(errorMsg, "ServiceUnavailable") ||