Switch x509_test.cc to modify the existing X509_VERIFY_PARAM.

There are two ways to configure an X509_STORE_CTX after
X509_STORE_CTX_init. One can either modify the already initialized
X509_VERIFY_PARAM or replace it. Modifying the existing one is more
common. Replacing it actually misses some defaults. (See issue #441 for
details.)

In preparation for actually being able to test changes to the default,
switch tests to that model. In doing so, no longer need to explicitly
configure the depth and can test that default. (Though we should write
tests for the depth at some point.)

Bug: 439, 441
Change-Id: I254a82585d70d44eb94920f604891ebfbff4af4c
Reviewed-on: https://boringssl-review.googlesource.com/c/boringssl/+/49745
Commit-Queue: David Benjamin <davidben@google.com>
Commit-Queue: Adam Langley <agl@google.com>
Reviewed-by: Adam Langley <agl@google.com>

crypto/x509/x509_test.cc

@@ -1146,19 +1146,14 @@ static int Verify(X509 *leaf, const std::vector<X509 *> &roots,
   X509_STORE_CTX_trusted_stack(ctx.get(), roots_stack.get());
   X509_STORE_CTX_set0_crls(ctx.get(), crls_stack.get());
 
-  X509_VERIFY_PARAM *param = X509_VERIFY_PARAM_new();
-  if (param == nullptr) {
-    return X509_V_ERR_UNSPECIFIED;
-  }
+  X509_VERIFY_PARAM *param = X509_STORE_CTX_get0_param(ctx.get());
   X509_VERIFY_PARAM_set_time(param, kReferenceTime);
-  X509_VERIFY_PARAM_set_depth(param, 16);
   if (configure_callback) {
     configure_callback(param);
   }
   if (flags) {
     X509_VERIFY_PARAM_set_flags(param, flags);
   }
-  X509_STORE_CTX_set0_param(ctx.get(), param);
 
   ERR_clear_error();
   if (X509_verify_cert(ctx.get()) != 1) {