Fix: update test certificate name. (#2300)

Thank you for opening a Pull Request! Before submitting your PR, there are a few things you can do to make sure it goes smoothly:
- [ ] Make sure to open an issue as a [bug/issue](https://github.com/googleapis/java-spanner/issues/new/choose) before writing your code!  That way we can discuss the change, evaluate designs, and agree on the general idea
- [ ] Ensure the tests and linter pass
- [ ] Code coverage does not decrease (if any source code was changed)
- [ ] Appropriate docs were updated (if necessary)

Fixes #<issue_number_goes_here> ☕️

If you write sample code, please follow the [samples format](
https://github.com/GoogleCloudPlatform/java-docs-samples/blob/main/SAMPLE_FORMAT.md).

Author: gyang-google

google-cloud-spanner-executor/src/main/java/com/google/cloud/executor/spanner/CloudUtil.java

@@ -46,7 +46,7 @@ public class CloudUtil {
   // error message causes the trailing headers to exceed this limit.
   private static final int GRPC_MAX_HEADER_LIST_SIZE_BYTES = 10 * 1024 * 1024;
 
-  private static final String TEST_HOST_IN_CERT = "test_cert_2";
+  private static final String TEST_HOST_IN_CERT = "test-cert-2";
 
   public static TransportChannelProvider newChannelProviderHelper(int port) {
     NettyChannelBuilder builder =