[x/sync] `TestFindNextKeyRandom` failure on `dev`

[patrick-ogrady]

-test.shuffle 1686710587734853800
--- FAIL: TestFindNextKeyRandom (0.29s)
    sync_test.go:481: seed: 1686710602115045300
    sync_test.go:680: 
        	Error Trace:	D:\a\avalanchego\avalanchego\x\sync\sync_test.go:680
        	Error:      	Not equal: 
        	            	expected: []byte{0x8a}
        	            	actual  : []byte{0x8a, 0x90}
        	            	
        	            	Diff:
        	            	--- Expected
        	            	+++ Actual
        	            	@@ -1,3 +1,3 @@
        	            	-([]uint8) (len=1) {
        	            	- 00000000  8a                                                |.|
        	            	+([]uint8) (len=2) {
        	            	+ 00000000  8a 90                                             |..|
        	            	 }
        	Test:       	TestFindNextKeyRandom
FAIL

Unclear if this is a bug in sync or just in the test. @StephenButtolph confirmed it fails reliably on the seed.

[StephenButtolph]

Fixed by #1624