Fix unittests on Windows

[MilesCranmer]

The unittests on master are currently failing on Windows (example), with the following unexpected pass:

 Unexpected Pass
 Expression: begin
    output = read(jlcmd, String)
    (sys_executable, exec_prefix, mod_file) = split(output, "\n")
    newpython == sys_executable
end
 Got correct result, please change to @test if no longer broken.

this PR changes @test_broken to @test as it seems to work now.

Also, this corrects the temporary base directory in the Windows tests to use the one specified by runner.temp, rather than that specified by %TEMP% which is what Base.mktempdir() uses.

(@stevengj this fixes the reason for the failures on #1015)

[codecov-commenter]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 68.36%. Comparing base (9f22684) to head (77115f5).
⚠️ Report is 22 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #1016      +/-   ##
==========================================
+ Coverage   68.35%   68.36%   +0.01%     
==========================================
  Files          20       20              
  Lines        2038     2039       +1     
==========================================
+ Hits         1393     1394       +1     
  Misses        645      645              

Flag	Coverage Δ
unittests	68.36% <ø> (+0.01%)	⬆️

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