Fix ZipValid test on non-English Windows systems

[HebaruSan]

Problem

If your Windows computer's locale is non-English, this test will fail:

1) Failed : Tests.Core.Cache.ZipValid_ContainsFilenameWithBadChars_NoException
  Expected string length 26 but was 27. Strings differ at index 7.
  Expected: "Illegales Zeichen im Pfad."
  But was:  "Illegal characters in path."
  ------------------^
   bei Tests.Core.Cache.ZipValid_ContainsFilenameWithBadChars_NoException() in C:\path\to\ckan-git\Tests\Core\Cache.cs:Zeile 195.

(I was able to reproduce this on an English computer by installing the German language pack for .NET and setting Thread.CurrentThread.CurrentUICulture = new CultureInfo("de-DE"); within the test.)

Cause

That error message is localized. It will appear in whatever language is selected in Thread.CurrentThread.CurrentUICulture.

Changes

Now we set Thread.CurrentThread.CurrentUICulture to CultureInfo.InvariantCulture for that test. This forces the error message to appear in English.

Fixes #2780.

[DasSkelett]

All tests passing again, great feeling :D

Test Run Summary
  Overall result: Passed
  Test Count: 685, Passed: 680, Failed: 0, Warnings: 0, Inconclusive: 0, Skipped: 5
    Skipped Tests - Ignored: 0, Explicit: 5, Other: 0
  Start time: 2019-06-12 20:59:16Z
    End time: 2019-06-12 21:00:03Z
    Duration: 46.986 seconds