spdm: Add mock-related macros

Add some handy macros to specify conditional compilation in test build
&& mock feature enabled.

BUG=b:284394614
TEST=spdm unittests

Change-Id: I5cf41b75d5f8acb6b2ab363574da83e4583ae5b3
Reviewed-on: https://chromium-review.googlesource.com/c/chromiumos/platform/spdm/+/5482366
Reviewed-by: Vadim Sukhomlinov <sukhomlinov@google.com>
Commit-Queue: Howard Yang <hcyang@google.com>
Tested-by: Howard Yang <hcyang@google.com>
6 files changed