server: fix TestLogGC test

This test started failing after #137069 was
merged into mater. That PR introduced a change
that made it so that a lock is acquired when
performing the initial scan for a DELETE,
over all scanned rows. As a result, a race
condition was exposed in TestLogGC (specifically
when the deadlock flag is set) where calling
`gcSystemLog` resulted in a
`TransactionRetryError: retry txn` error.

This race condition is happening because the
test data being written is using timestamps
in the future as opposed to the past. When
attemping to GC the table, its conflict with
normal writes that are happening at the
same time. To fix, the test data is being
written in the past, which avoids these
conflicts.

Fixes: #137490
Release note: None

pkg/server/server_systemlog_gc_test.go

@@ -67,7 +67,7 @@ func TestLogGC(t *testing.T) {
 			a.NoError(err)
 		}
 	}
-	maxTS1 := timeutil.Now()
+	maxTS1 := timeutil.Now().Add(time.Duration(-24*7) * time.Hour)
 	maxTS2 := maxTS1.Add(time.Second)
 	maxTS3 := maxTS2.Add(time.Second)
 	maxTS4 := maxTS3.Add(time.Second)