net/netcheck: don't mutate Client.TimeNow in AddReportHistoryForTest

AddReportHistoryForTest temporarily swapped out Client.TimeNow without
holding any lock, racing with concurrent GetReport calls reading it
from ReSTUN goroutines during tests. Instead, pass the current time to
addReportHistoryAndSetPreferredDERP explicitly so the test helper never
needs to touch the field.

Fixes #20438

Signed-off-by: Brad Fitzpatrick <bradfitz@tailscale.com>
Change-Id: I9c4f0a2f9427b5f1d3e8b06a49f0d2b71c3ee8a4

net/netcheck/netcheck_test.go

@@ -468,7 +468,7 @@ func TestAddReportHistoryAndSetPreferredDERP(t *testing.T) {
 			for _, s := range tt.steps {
 				fakeTime = fakeTime.Add(s.after)
 				rs.start = fakeTime.Add(-100 * time.Millisecond)
-				c.addReportHistoryAndSetPreferredDERP(rs, s.r, dm.View())
+				c.addReportHistoryAndSetPreferredDERP(rs, s.r, dm.View(), fakeTime)
 			}
 			lastReport := tt.steps[len(tt.steps)-1].r
 			if got, want := len(c.prev), tt.wantPrevLen; got != want {
@@ -522,7 +522,7 @@ func TestRecentReportsRetainFullNetcheck(t *testing.T) {
 			regions = allRegions
 			lastFull = now
 		}
-		c.addReportHistoryAndSetPreferredDERP(&reportState{c: c, start: now}, mkReport(regions), dm.View())
+		c.addReportHistoryAndSetPreferredDERP(&reportState{c: c, start: now}, mkReport(regions), dm.View(), now)
 
 		// Recent latency must always cover every region, which is only
 		// possible while a full report remains in c.prev.