Adding function to set the "current time" callback used for DTLS.

This callback is used by BoringSSL tests in order to simulate the time,
so that the tests have repeatable results. This API will allow consumers
of BoringSSL to write the same sort of tests.

Change-Id: I79d72bce5510bbd83c307915cd2cc937579ce948
Reviewed-on: https://boringssl-review.googlesource.com/8200
Reviewed-by: David Benjamin <davidben@google.com>

ssl/ssl_lib.c

diff --git a/ssl/ssl_lib.c b/ssl/ssl_lib.c
index 53dc2e5..7f7bf5a 100644
--- a/ssl/ssl_lib.c
+++ b/ssl/ssl_lib.c
@@ -2356,6 +2356,12 @@
   ctx->keylog_callback = cb;
 }
 
+void SSL_CTX_set_current_time_cb(SSL_CTX *ctx,
+                                 void (*cb)(const SSL *ssl,
+                                            struct timeval *out_clock)) {
+  ctx->current_time_cb = cb;
+}
+
 static int cbb_add_hex(CBB *cbb, const uint8_t *in, size_t in_len) {
   static const char hextable[] = "0123456789abcdef";
   uint8_t *out;