x/net/webdav: TestDir fails on nacl

[mikioh]

See http://build.golang.org/log/db23eb1244d8fd6bd544d395413cd0c700a7f6be.

--- FAIL: TestDir (0.01s)
    file_test.go:503: test case #61 "move__ o=F /d /d/n/z want err": got "ok" (<nil>), want "err"
FAIL
FAIL    golang.org/x/net/webdav 0.971s

[gopherbot]

CL https://golang.org/cl/13055 mentions this issue.

[bradfitz]

Closing nacl bugs, as we're removing nacl (#30439)