Make SystemTime mockable #5169

Job	Run time
test on solaris	6h 0m 1s
test on freebsd	4m 16s
test on netbsd	5m 16s
test on illumos	6m 54s
audit	22s
features (ubuntu-latest)	1h 22m 20s
lint	40s
msrv	59s
features (macos-latest)	25m 29s
test (ubuntu-latest, stable)	1m 30s
features (windows-latest)	50m 49s
test-android (x86_64-linux-android, x86_64, 26)	2m 58s
test-aws-lc-rs	1m 53s
test (ubuntu-latest, beta)	1m 25s
test-android (i686-linux-android, x86, 26)	2m 54s
test wasm32-unknown-unknown	55s
test (macos-latest, stable)	1m 5s
test (windows-latest, stable)	3m 48s
	9h 13m 34s

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