ssl_test_lib: move initial includes and is-it-supported check

Move from ssl_*2.c to ssl_test_lib.h:
* The inclusion of the config header file (identical).
* The fallback definitions of platform functions (almost identical:
  server2 also had snprintf and the order and whitespace was slightly
  different).
* The compilation guards checking that required options are
  enabled (tweaked to make them common, with MBEDTLS_SSL_{CLI,SRV}_C
  separated out).

Signed-off-by: Gilles Peskine <Gilles.Peskine@arm.com>
diff --git a/programs/ssl/ssl_test_lib.c b/programs/ssl/ssl_test_lib.c
index 897afc6..40a6aa9 100644
--- a/programs/ssl/ssl_test_lib.c
+++ b/programs/ssl/ssl_test_lib.c
@@ -22,3 +22,6 @@
 
 #include "ssl_test_lib.h"
 
+#if !defined(MBEDTLS_SSL_TEST_IMPOSSIBLE)
+
+#endif /* !defined(MBEDTLS_SSL_TEST_IMPOSSIBLE) */