test: cope with uutils coreutils flag parsing for date command

With GNU coreutils, one can pass an argument to the "set" flag like:

 date -s "-3 days"

With uutils coreutils, that gives an error:

 error: unexpected argument '-3' found
    tip: to pass '-3' as a value, use '-- -3'

That's a bug (https://github.com/uutils/coreutils/issues/9679), but both
implementations accept --set="-3 days" just fine, so use that instead.

test/units/TEST-53-TIMER.RandomizedDelaySec-reload.sh

@@ -84,7 +84,7 @@ check_elapse_timestamp
 # elapse timestamp (this goes through a slightly different codepath that actually contained the original
 # issue).
 : "Next elapse timestamp after time jump"
-date -s "tomorrow 00:10"
+date --set="tomorrow 00:10"
 check_elapse_timestamp
 
 : "Next elapse timestamp after daemon-reload"

test/units/TEST-53-TIMER.issue-16347.sh

@@ -4,8 +4,8 @@ set -eux
 set -o pipefail
 
 # Reset host date to current time, 3 days in the past.
-date -s "-3 days"
-trap 'date -s "+3 days"' EXIT
+date --set="-3 days"
+trap 'date --set="+3 days"' EXIT
 
 # Run a timer for every 15 minutes.
 systemd-run --unit test-timer --on-calendar "*:0/15:0" true

test/units/TEST-53-TIMER.restart-trigger.sh

@@ -37,8 +37,8 @@ JOURNAL_TS="$(date "+%s")"
 systemctl restart "$UNIT_NAME.timer"
 systemctl status "$UNIT_NAME.timer"
 
-date -s '+2 hours'
-trap 'date -s "-2 hours"' EXIT
+date --set='+2 hours'
+trap 'date --set="-2 hours"' EXIT
 sleep 1
 systemctl status "$UNIT_NAME.timer"
 assert_eq "$(journalctl -q -p info --since="@$JOURNAL_TS" --unit="$UNIT_NAME" --grep="$TEST_MESSAGE" | wc -l)" "1"