test/repl: try to fix mkdir issue on FreeBSD CI

[iblislin]

Not sure why FreeBSD CI ran into the mkdir issue several times.

Error in testset REPL:
Error During Test at /usr/home/julia/julia-fbsd-buildbot/worker/11rel-amd64/build/test/testdefs.jl:19
  Got exception LoadError("/usr/home/julia/julia-fbsd-buildbot/worker/11rel-amd64/build/usr/share/julia/stdlib/v0.7/REPL/test/runtests.jl", 7, LoadError("/usr/home/julia/julia-fbsd-buildbot/worker/11rel-amd64/build/usr/share/julia/stdlib/v0.7/REPL/test/replcompletions.jl", 593, SystemError("mkdir", 17, "/tmp/replcompletion-unreadable"))) outside of a @test
  LoadError: LoadError: SystemError (with /tmp/replcompletion-unreadable): mkdir: File exists
  Stacktrace:
   [1] #systemerror#43(::String, ::Function, ::Symbol, ::Bool) at ./error.jl:106
   [2] #systemerror at ./none:0 [inlined]
   [3] #mkdir#7(::UInt16, ::Function, ::String) at ./file.jl:167
   [4] mkdir(::String) at ./file.jl:162
...

• https://freebsdci.julialang.org/#/builders/1/builds/10091
• https://freebsdci.julialang.org/#/builders/1/builds/10089
• https://freebsdci.julialang.org/#/builders/1/builds/10088
• https://freebsdci.julialang.org/#/builders/1/builds/10086
• https://freebsdci.julialang.org/#/builders/1/builds/10083

[iblislin]

Seems there are lots of leftovers dirs and files on my one of works, also.

┌─[/tmp]
└─[iblis@abeing]% ls -l | wc -l
    1598

[staticfloat]

LGTM

[ararslan]

AppVeyor failures are timeouts, macOS failure is unrelated (FileWatching).