Improve testing switch usage in build script
Improve testing switch usage in build script

When .\build.ps1 -Test is given, it runs engine and rules tests. Now if -Engine switch is added to previous command, only engine tests are run and so for rules. When -RunInDifferentProcess sitch is given, it opens another PS console and runs the tests there.

juneb/PSScriptAnalyzer,dlwyatt/PSScriptAnalyzer,PowerShell/PSScriptAnalyzer,daviwil/PSScriptAnalyzer