Use the -allow-unimplemented feature of the BoringSSL runner

That way we can remove flags that we don't support
Reviewed-by: NRichard Levitte <levitte@openssl.org>

test/ossl_shim/test_config.cc

@@ -191,6 +191,7 @@ bool ParseConfig(int argc, char **argv, TestConfig *out_config) {
     }
 
     fprintf(stderr, "Unknown argument: %s\n", argv[i]);
+    exit(89);
     return false;
   }
 

test/recipes/90-test_external.t

@@ -24,7 +24,7 @@ indir $ENV{BORING_RUNNER_DIR} => sub {
                       srctop_file("test", "ossl_shim", "ossl_shim"),
                       "-shim-config",
                       srctop_file("test", "ossl_shim", "ossl_config.json"),
-                      "-pipe"])),
+                      "-pipe", "-allow-unimplemented"])),
         "running external tests");
 }, create => 0, cleanup => 0;