Clarify that the "FATAL" message is expected

The test case "Memory buffer small buffer" emits a message
"FATAL: verification of first header failed". In this test case, it's
actually expected, but it looks weird to see this message from a
passing test. Add a comment that states this explicitly, and modify
the test description to indicate that the failure is expected, and
change the test function name to be more accurate.

Fix #309

Author: gilles-peskine-arm

tests/suites/test_suite_memory_buffer_alloc.data

@@ -16,8 +16,8 @@ memory_buffer_alloc_free_alloc:100:64:100:100:0:0:0:1:200:0
 Memory buffer alloc - Out of Memory test
 memory_buffer_alloc_oom_test:
 
-Memory buffer small buffer
-memory_buffer_small_buffer:
+Memory buffer: heap too small (header verification should fail)
+memory_buffer_heap_too_small:
 
 Memory buffer underalloc
 memory_buffer_underalloc:

tests/suites/test_suite_memory_buffer_alloc.function

@@ -232,11 +232,14 @@ exit:
 /* END_CASE */
 
 /* BEGIN_CASE depends_on:MBEDTLS_MEMORY_DEBUG */
-void memory_buffer_small_buffer( )
+void memory_buffer_heap_too_small( )
 {
     unsigned char buf[1];
 
     mbedtls_memory_buffer_alloc_init( buf, sizeof( buf ) );
+    /* With MBEDTLS_MEMORY_DEBUG enabled, this prints a message
+     * "FATAL: verification of first header failed".
+     */
     TEST_ASSERT( mbedtls_memory_buffer_alloc_verify() != 0 );
 }
 /* END_CASE */