Create test/testutil.h for unit test helper macros

Defines SETUP_TEST_FIXTURE and EXECUTE_TEST, and updates ssl/heartbeat_test.c
using these macros. SETUP_TEST_FIXTURE makes use of the new TEST_CASE_NAME
macro, defined to use __func__ or __FUNCTION__ on platforms that support those
symbols, or to use the file name and line number otherwise. This should fix
several reported build problems related to lack of C99 support.
test/README and test template generator