accounts/keystore: add sleep back for TestUpdatedKeyfileContents

[darioush]

Hi. I understand that in #28461 these sleeps were replaced with Chtimes to reduce test execution time.
However, I noticed this test sometimes fails on linux with the race detector enabled, eg:
go test -timeout 900s github.com/ethereum/go-ethereum/accounts/keystore -cover -count=10 -race -shuffle=on

I believe updating the mod time to a past time does not work due to the way the fileCache tracks lastMod:

go-ethereum/accounts/keystore/file_cache.go

Lines 71 to 77 in 9dcf8aa

	modified := info.ModTime()
	if modified.After(fc.lastMod) {
	mods.Add(path)
	}
	if modified.After(newLastMod) {
	newLastMod = modified
	}

The sleeps seem preferable to modifying the file cache for testing, or alternatively resetting the lastMod field in the test. Thanks for your consideration.

[fjl]

Test was fixed in #29867