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

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 <cyphar@cyphar.com>

util_linux_test.go

@@ -11,6 +11,7 @@ import (
 	"fmt"
 	"os"
 	"path/filepath"
+	"runtime"
 	"strconv"
 	"strings"
 	"testing"
@@ -111,6 +112,8 @@ func doRenameExchangeLoop(pauseCh chan struct{}, exitCh <-chan struct{}, dir *os
 					// enter a bad filesystem state if we get paused.
 					panic(fmt.Sprintf("renameat2([%d]%q, %q, ..., %q, RENAME_EXCHANGE) = %v", int(dir.Fd()), dir.Name(), pathA, pathB, err))
 				}
+				// Make sure GC doesn't close the directory handle.
+				runtime.KeepAlive(dir)
 			}
 		}
 	}