test: update proto columns test (#2845)

* test(spanner): add try-with-resources block to free up resources

* test(spanner): add test for to string

* test(spanner): fix tests

Author: harshachinta

google-cloud-spanner/src/main/java/com/google/cloud/spanner/Value.java

@@ -1749,9 +1749,15 @@ com.google.protobuf.Value valueToProto() {
       return com.google.protobuf.Value.newBuilder().setStringValue(base64EncodedString).build();
     }
 
+    @Nonnull
+    @Override
+    public String getAsString() {
+      return value == null ? NULL_STRING : value.toBase64();
+    }
+
     @Override
     void valueToString(StringBuilder b) {
-      b.append(value.toString());
+      b.append(value.toBase64());
     }
   }
 
@@ -2387,7 +2393,7 @@ String elementToString(ByteArray element) {
 
     @Override
     void appendElement(StringBuilder b, ByteArray element) {
-      b.append(element.toString());
+      b.append(element.toBase64());
     }
   }