fix(test): Make check_client_subscriptions.c time-deterministic #1030

Job	Run time
Debug Build & Unit Tests (gcc)	4m 5s
Debug Build & Unit Tests (gcc, 32bit)	3m 49s
Debug Build & Unit Tests without Subscriptions (gcc)	2m 8s
Debug Build & Unit Tests with Diagnostics (gcc)	3m 29s
Debug Build & Unit Tests with multithreading (gcc)	2m 30s
Debug Build & Unit Tests with Alarms&Conditions (gcc)	12m 31s
Debug Build & Unit Tests (clang)	5m 23s
Debug Build & Unit Tests (tcc)	2m 39s
Encryption (MbedTLS) Build & Unit Tests (gcc)	3m 13s
PubSub Encryption (MbedTLS) Build & Unit Tests (gcc)	3m 43s
PubSub SKS Build & Unit Tests (gcc)	5m 14s
Encryption (OpenSSL) Build & Unit Tests (gcc)	2m 44s
Encryption (LibreSSL) Build & Unit Tests (gcc)	6m 20s
TPM Tool Build	3m 5s
Release Build	1m 33s
Amalgamation Build	1m 29s
Amalgamation Build with Multithreading	1m 8s
Valgrind Build & Unit Tests with MbedTLS (gcc)	17m 9s
Valgrind Build & Unit Tests with OpenSSL (gcc)	17m 9s
Valgrind Examples with MbedTLS (gcc)	1h 9m 38s
Valgrind Examples with OpenSSL (gcc)	1h 15m 7s
Clang Static Analyzer	5m 6s
Build All Companion Specifications	18m 20s
	4h 27m 32s

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