Windows: temp file names too short

[StefanKarpinski]

From https://build.julialang.org/#/builders/32/builds/4736/steps/2/logs/stdio for example:

Error in testset Test:
Error During Test at C:\cygwin\home\Administrator\buildbot\worker-tabularasa\tester_win64\build\share\julia\stdlib\v1.4\Test\test\runtests.jl:863
  Got exception outside of a @test
  SystemError: opening file "C:\\cygwin\\tmp\\jl_6E.tmp": No such file or directory

With only two characters in the random temp file name, there are going to be a lot of collisions.

[vtjnash]

There's actually 4 characters (65k values), but seems like we should take Jeff's advice? from #9053 (comment)

[StefanKarpinski]

How are there 4 characters? jl_6E.tmp has two variable characters as most.

[JeffBezanson]

I think this is fixed.