Describe FIPS deferred tests implementation

Describe FIPS deferred tests implementation

Add a "Current Implementation" section to the FIPS deferred tests design
document.

This section details the mechanism used to execute on-demand self-tests in a
thread-safe manner. It explains the roles of the core functions, the use of a
global lock to serialize test execution, and the thread-local storage strategy
to prevent deadlocks during nested test calls.

Signed-off-by: Simo Sorce <simo@redhat.com>
Reviewed-by: Tomas Mraz <tomas@openssl.org>
Reviewed-by: Dmitry Belyavskiy <beldmit@gmail.com>
Reviewed-by: Paul Dale <paul.dale@oracle.com>
(Merged from https://github.com/openssl/openssl/pull/29004)