Fix relative paths for OutputXml

The xmlwriter method used to output nUnit report can resolve relatives
paths by itself, but its current directory is not necessarily the same
as what PowerShell sees as the current directory. I have to resolve the
path beforehand working around the limitations of Resolve-Path.
Remove Validate-Xml

Removing validate xml function which was replaced in the tests with less
general way of validating nUnit output xml files.
Remove legacy object adaptations support

This has been long talked about and the new 3.0.0 release is the ideal
release to finally get rid of the legacy way of using pester.
Remove tests testing usage of global scope

Removed the unused code that used to test the usage of global scope.
Looking through the code the only global variable is the global
TestDrive variable. This variable is provided for backwards
compatibility.

$pesterHome = Get-Module Pester | Select -ExpandProperty Path |
Split-Path
Get-ChildItem -Path $pesterHome -Include *.ps1, *.psm1 -Recurse |
Get-Content |
Select-String -Pattern '(\$global)|(-Scope\s+Global)'

Testing this properly needs running pester inside pester tests (not sure
if the whole test suite should be run or just a single test is enough)
and comparing the state of the global scope before and after run. Making
mental note of this and moving it to low priority folder.