Add SSL_CTX_get_num_tickets.

CPython uses this function. Our implementation is slightly weird since
it leaks the clamping behavior, but probably not a big deal.

Update-Note: When this is merged into the internal repository, we can
simplify the CPython patches.

Change-Id: I291ddf852fb463bf02998fe04d0d0e8cb358dc55
Reviewed-on: https://boringssl-review.googlesource.com/c/boringssl/+/53485
Commit-Queue: Bob Beck <bbe@google.com>
Auto-Submit: David Benjamin <davidben@google.com>
Commit-Queue: David Benjamin <davidben@google.com>
Reviewed-by: Bob Beck <bbe@google.com>

ssl/ssl_test.cc

diff --git a/ssl/ssl_test.cc b/ssl/ssl_test.cc
index fe85840..6bc015c 100644
--- a/ssl/ssl_test.cc
+++ b/ssl/ssl_test.cc
@@ -8151,11 +8151,13 @@
   for (size_t num_tickets : {0, 1, 2, 3, 4, 5}) {
     SCOPED_TRACE(num_tickets);
     ASSERT_TRUE(SSL_CTX_set_num_tickets(server_ctx.get(), num_tickets));
+    EXPECT_EQ(SSL_CTX_get_num_tickets(server_ctx.get()), num_tickets);
     EXPECT_EQ(count_tickets(), num_tickets);
   }
 
   // Configuring too many tickets causes us to stop at some point.
   ASSERT_TRUE(SSL_CTX_set_num_tickets(server_ctx.get(), 100000));
+  EXPECT_EQ(SSL_CTX_get_num_tickets(server_ctx.get()), 16u);
   EXPECT_EQ(count_tickets(), 16u);
 }