[tests] Export $USE_MSBUILD=$(USE_MSBUILD)

[jonpryor]

Context: 286b9c2
Context: https://jenkins.mono-project.com/view/Xamarin.Android/job/xamarin-android/700/testReport/

Xamarin.ProjectTools was updated in commit 286b9c2 to use msbuild
(via xabuild) when $USE_MSBUILD is not 0, and to continue
using xbuild (the historical default!) when $USE_MSBUILD is 0.

Unfortunately, nothing exports $USE_MSBUILD. Consequently, we
always use msbuild/xabuild, unless the calling environment
explicitly exports it (which isn't the case on Jenkins).

Update the RUN_NUNIT_TEST make define so that the $USE_MSBUILD
environment variable is exported to the value of the
$(USE_MSBUILD) make variable, IFF it's not the empty string.
If $(USE_MSBUILD) is the empty string, export USE_MSBUILD=0,
which will cause xbuild to be used.

With luck, this will allow the current Xamarin.Android.Build.Tests
failures -- all 20 of them -- to be "fixed" until such time as all
Xamarin.Android.Build.Tests tests actually pass w/ MSBuild.

[dellis1972]

This should not be needed..

RunningMSBuild = true;
if (!string.IsNullOrEmpty (useMSBuild) && useMSBuild == "0") {
    RunningMSBuild = false;
}

if the USE_MSBUILD is empty this will fail and we will continue to use msbuild which I thought was the goal? This PR will force us back to using xbuild for the tests..