Ignore old -enable-ed25519 flag.

Change 1766935f76 removed this flag but it's useful if bssl_shim ignores
it to reduce noise in cross-version testing. This can be dropped in
three months once the old versions have aged out.

Change-Id: I73f2bebeb5e8c178253fbb6915026e06b6ad58bc
Reviewed-on: https://boringssl-review.googlesource.com/c/boringssl/+/40084
Commit-Queue: Adam Langley <agl@google.com>
Commit-Queue: David Benjamin <davidben@google.com>
Reviewed-by: David Benjamin <davidben@google.com>

ssl/test/test_config.cc

diff --git a/ssl/test/test_config.cc b/ssl/test/test_config.cc
index c7be4a4..f497704 100644
--- a/ssl/test/test_config.cc
+++ b/ssl/test/test_config.cc
@@ -307,6 +307,12 @@
     return true;
   }
 
+  if (strcmp(flag, "-enable-ed25519") == 0) {
+    // Old argument; ignored for split-handshake compat testing.
+    // Remove after 2020-06-01.
+    return true;
+  }
+
   fprintf(stderr, "Unknown argument: %s.\n", flag);
   return false;
 }