tests: make sure the directory handle is kept alive in rename loop

[cyphar]

It turns out the GC can close the directory during our loop in CI sometimes, leading to the following failures (note that dir.Fd() returns -1, which is a closed directory):

  === RUN   TestPartialLookup_RacingRename/openat2=true/swap-dir-danglinglink-basic
  panic: renameat2([-1]"/tmp/TestPartialLookup_RacingRenameopenat2=trueswap-dir-file-basic3291115487/001/tree", "a/b", ..., "file", RENAME_EXCHANGE) = no such file or directory

  goroutine 552 [running]:
  github.com/cyphar/filepath-securejoin.doRenameExchangeLoop(0xc000027680, 0xc0000276e0, 0xc0000025a8, {0x66c786, 0x3}, {0x66c8f4, 0x4})
      /home/runner/work/filepath-securejoin/filepath-securejoin/util_linux_test.go:111 +0x38c
  created by github.com/cyphar/filepath-securejoin.TestPartialLookup_RacingRename.func1.1 in goroutine 551
      /home/runner/work/filepath-securejoin/filepath-securejoin/lookup_linux_test.go:561 +0x525
  exit status 2

Signed-off-by: Aleksa Sarai [email protected]