Add platform setup and teardown calls to test suites

Add a global platform context variable available for tests
diff --git a/tests/suites/helpers.function b/tests/suites/helpers.function
index eef41c7..9295bfa 100644
--- a/tests/suites/helpers.function
+++ b/tests/suites/helpers.function
@@ -109,6 +109,7 @@
 }
 test_info;
 
+mbedtls_platform_context platform_ctx;
 
 /*----------------------------------------------------------------------------*/
 /* Helper flags for complex dependencies */