Panic: unaligned 64-bit atomic operation

[gibmat]

When built on a 32-bit architecture, the tests fail with unaligned panics:

----------------------------------------------------------------------
PANIC: dilated_test.go:93: dilatedClockSuite.TestAdvance

... Panic: unaligned 64-bit atomic operation (PC=0x807FAA2)

/usr/lib/go-1.18/src/runtime/panic.go:838
  in gopanic
/usr/lib/go-1.18/src/runtime/internal/atomic/unaligned.go:8
  in panicUnaligned
/usr/lib/go-1.18/src/runtime/internal/atomic/atomic_386.s:225
  in Load64
dilated.go:50
  in dilationClock.nowWithOffset
dilated.go:114
  in dilatedWallTimer.start
dilated.go:108
  in newDilatedWallTimer
dilated.go:58
  in dilationClock.After
dilated_test.go:96
  in dilatedClockSuite.TestAdvance
/usr/lib/go-1.18/src/reflect/value.go:339
  in Value.Call
/usr/lib/go-1.18/src/runtime/asm_386.s:1326
  in goexit

The following patch fixes the issue, by ensuring that the variable used in atomic operations is at the beginning of the struct to guarantee proper alignment:

diff --git a/testclock/dilated.go b/testclock/dilated.go
index 4333b11..0b4d764 100644
--- a/testclock/dilated.go
+++ b/testclock/dilated.go
@@ -24,11 +24,14 @@ func NewDilatedWallClock(realSecondDuration time.Duration) AdvanceableClock {
 }
 
 type dilationClock struct {
+	// offsetAtomic is the current dilated offset to allow for time jumps/advances.
+	// Must be the first item in the struct to ensure proper 64-bit alignment as
+	// documented at https://pkg.go.dev/sync/atomic#pkg-note-BUG
+	offsetAtomic int64
+
 	epoch              time.Time
 	realSecondDuration time.Duration
 
-	// offsetAtomic is the current dilated offset to allow for time jumps/advances.
-	offsetAtomic int64
 	// offsetChanged is a channel that is closed when timers need to be signaled
 	// that there is a offset change coming.
 	offsetChanged chan any