Tests: Replace %TEMP% path hardcodings

[savornicesei]

There are several places in unit tests where temporary path is hardcoded, for example:

• project/UnitTests/Core/Config/PreprocessorTest.cs FAKE_ROOT
• project/UnitTests/Core/IntegrationResultManagerTest.cs Assert.AreEqual(Platform.IsWindows ? @"c:\temp" : @"/tmp", result.WorkingDirectory);
• project/UnitTests/Core/ProjectTest.cs Assert.AreEqual(Platform.IsWindows ? @"c:\temp" : @"/tmp", result.WorkingDirectory);

We can use Path.GetTempPath or the env. variables (see the remarks section in the link) to get the temp path without checking the platform.