test: don't timeout while waiting for other test units

The main testsuite service timeouts sporadically when waiting for
other testsuite-* units. As the test timeout is handled by
the "test executor" (test.sh), let's disable it for the service.

This should (hopefully) fix the test flakiness.

(cherry picked from commit 772f15c)

test/TEST-16-EXTEND-TIMEOUT/testsuite.service

@@ -12,7 +12,7 @@ Wants=testsuite-fail-runtime.service
 StopWhenUnneeded=yes
 
 [Service]
-
 Type=simple
+TimeoutStartSec=infinity
 ExecStartPre=/assess.sh
 ExecStart=/bin/true