test: Stop assuming RTC module in testbed kernel
test: Stop assuming RTC module in testbed kernel

Setting the time to year 2038 will not persist across reboots if the
testbed kernel does not have RTC enabled. This is the case on e. g. the
Debian cloud kernel.

Thus set the time back to the expected time after rebooting the test VM.

Closes #12672
mvollmer/cockpit,deryni/cockpit,deryni/cockpit,cockpit-project/cockpit,moraleslazaro/cockpit,garrett/cockpit,mvollmer/cockpit,mvollmer/cockpit,cockpituous/cockpit,martinpitt/cockpit,deryni/cockpit,moraleslazaro/cockpit,deryni/cockpit,moraleslazaro/cockpit,mvollmer/cockpit,martinpitt/cockpit,cockpituous/cockpit,moraleslazaro/cockpit,cockpit-project/cockpit,cockpituous/cockpit,martinpitt/cockpit,cockpit-project/cockpit,garrett/cockpit,cockpituous/cockpit,garrett/cockpit,cockpit-project/cockpit,cockpituous/cockpit,deryni/cockpit,cockpituous/cockpit,moraleslazaro/cockpit,deryni/cockpit,mvollmer/cockpit,garrett/cockpit,cockpit-project/cockpit,martinpitt/cockpit,deryni/cockpit,martinpitt/cockpit,moraleslazaro/cockpit,moraleslazaro/cockpit,garrett/cockpit