okhttp: Use normal server cert when testing trust checking

Previously, untrustedServer_fails could have been failing for the same
reason as unmatchedServerSubjectAlternativeNames_fails. The
implementation could have been broken and not checking the cert chain
but still checking the hostname. We'd either need to override the
authority to match the badserver cert or use the normal server
certificates. It is best to use the normal server certificates as
mtls_succeeds confirms the configuration is correct and so our test is
failing for the right reason.
diff --git a/okhttp/src/test/java/io/grpc/okhttp/TlsTest.java b/okhttp/src/test/java/io/grpc/okhttp/TlsTest.java
index cc86c81..9834d52 100644
--- a/okhttp/src/test/java/io/grpc/okhttp/TlsTest.java
+++ b/okhttp/src/test/java/io/grpc/okhttp/TlsTest.java
@@ -173,8 +173,8 @@
   @Test
   public void untrustedServer_fails() throws Exception {
     ServerCredentials serverCreds;
-    try (InputStream serverCert = TlsTesting.loadCert("badserver.pem");
-         InputStream serverPrivateKey = TlsTesting.loadCert("badserver.key");
+    try (InputStream serverCert = TlsTesting.loadCert("server1.pem");
+         InputStream serverPrivateKey = TlsTesting.loadCert("server1.key");
          InputStream caCert = TlsTesting.loadCert("ca.pem")) {
       serverCreds = TlsServerCredentials.newBuilder()
           .keyManager(serverCert, serverPrivateKey)
@@ -183,11 +183,9 @@
     }
     ChannelCredentials channelCreds;
     try (InputStream clientCertChain = TlsTesting.loadCert("client.pem");
-         InputStream clientPrivateKey = TlsTesting.loadCert("client.key");
-         InputStream caCert = TlsTesting.loadCert("ca.pem")) {
+         InputStream clientPrivateKey = TlsTesting.loadCert("client.key")) {
       channelCreds = TlsChannelCredentials.newBuilder()
           .keyManager(clientCertChain, clientPrivateKey)
-          .trustManager(caCert)
           .build();
     }
     Server server = grpcCleanupRule.register(server(serverCreds));