Revert "Remove tests that depend on TLS or X.509"

This reverts commit 9afb2e992136db3fae9a669c3faaf6d5d27602a8.

Conflicts:
* include/CMakeLists.txt
  * "Make config.h available" comment: there has been a change
    adjacent to where it was removed. Just re-add what was removed.
* tests/CMakeLists.txt:
  * compat.sh: there has been a change immediately before where it was
    removed. Just re-add what was removed.
diff --git a/configs/config-thread.h b/configs/config-thread.h
index f729a03..25db16b 100644
--- a/configs/config-thread.h
+++ b/configs/config-thread.h
@@ -75,6 +75,10 @@
 #define MBEDTLS_SSL_SRV_C
 #define MBEDTLS_SSL_TLS_C
 
+/* For tests using ssl-opt.sh */
+#define MBEDTLS_NET_C
+#define MBEDTLS_TIMING_C
+
 /* Save RAM at the expense of ROM */
 #define MBEDTLS_AES_ROM_TABLES