NUnit.MSBuild Should Optionally Allow For Passing the Format Argument

[aolszowka]

Several Products (such as BambooCI and CruiseControl.NET) DO NOT support the nunit3 log result type.

To work around this we should optionally allow for the format=nunit2 argument to be passed though to NUnit3 Console as per https://docs.nunit.org/articles/nunit/running-tests/Console-Command-Line.html

Several options that specify processing of XML output take an output specification as a value. A SPEC may take one of the following forms:

    --OPTION:filename
    --OPTION:filename;format=formatname
    --OPTION:filename;transform=xsltfile

The --result option may use any of the following formats:

    nunit3 - the native XML format for NUnit 3
    nunit2 - legacy XML format used by earlier releases of NUnit