Make SystemTime mockable via object #5218

Job	Run time
test on illumos	6m 54s
test on freebsd	3m 52s
test (ubuntu-latest, stable)	1m 31s
test-aws-lc-rs	1m 51s
test on netbsd	5m 13s
test (ubuntu-latest, beta)	1m 17s
test on solaris	15m 8s
test (macos-latest, stable)	1m 14s
test (windows-latest, stable)	4m 8s
audit	24s
features (ubuntu-latest)	1h 23m 48s
lint	1m 8s
features (macos-latest)	30m 3s
msrv	58s
features (windows-latest)	49m 0s
test-android (x86_64-linux-android, x86_64, 26)	2m 57s
test wasm32-unknown-unknown	48s
test-android (i686-linux-android, x86, 26)	3m 1s
	3h 33m 15s

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