Create ssl_helpers.c to hold functions of TLS connection

test_suite_ssl.function contains many functions that are used to set
up a TLS connection. To reduce its file size, those functions would
be moved to ssl_helpers.c under tests/src. As the start of this
implementation, some necessary header files are moved in advance.

Signed-off-by: Yanray Wang <yanray.wang@arm.com>
diff --git a/tests/suites/test_suite_ssl.function b/tests/suites/test_suite_ssl.function
index 39825ed..4657df5 100644
--- a/tests/suites/test_suite_ssl.function
+++ b/tests/suites/test_suite_ssl.function
@@ -7,6 +7,7 @@
 #include <mbedtls/timing.h>
 #include <mbedtls/debug.h>
 #include <ssl_tls13_keys.h>
+#include <test/ssl_helpers.h>
 
 #include <constant_time_internal.h>