[Xamarin.Android.Build.Tasks] First Pass at adding Unit tests for the MSBuild Tasks

[dellis1972]

This commit adds basic support for the MSBuild Unit Tests. The goal
of these tests is to ensure that the build of an android applicaition
works consistently on all the supported platforms.

The basic layout of a Unit test is as follows

[Test]
public void BuildReleaseApplication ()
{
    var proj = new XamarinAndroidApplicationProject () {
        IsRelease = true,
    };
    using (var b = CreateApkBuilder (Path.Combine ("temp", TestContext.CurrentContext.Test.Name))) {
        Assert.IsTrue (b.Build (proj), "Build should have succeeded.");
    }
}

It is a standard Unit test. First we create a XamarinAndroidApplicatonProject
and set its properties. You can use the proj to add new source files,
references, assets, resources and nuget packages. By default you will get
a standard "HelloWorld" android application.
Once you have the project you will need to create either a Apk or Dll builder
via the helper methods

CreateApkBuilder
CreateDllBuilder

CreateApkBuilder will create an apk and by default will execute the
SignAndroidPackage build target. the CreateDllBuilder will produce
a dll. The source files are created in a temp directory relative to
the build output of the unit tests. By default this is

src/Xamarin.Android.Build.Tasks/UnitTests/Xamarin.Android.Build.Tests/bin/$(Configuraiton)/temp

Once you have a builder you can then call Build passing in the project. There are
also methods available for Clean and Save. Running any of these will cause
the Unit test to shell out to xbuild/msbuild and attempt to build the project.
Test results are written to a build.log file.
If a unit test passes then the test directory is removed. If a test fails
the directory and the build.log will be left in place for investigation.

[dellis1972]

@jonpryor don't merge this yet (if you are tempted) I still need to update it to use the xabuild script rather than system xbuild. Also it needs updating to look for the built framework.

[jonpryor]

+++ src/Xamarin.Android.Build.Tasks/UnitTests/Xamarin.Android.Build.Tests/BuildTest.cs

Convention is that unit tests go into a nested Test directory, or Tests (oops), not UnitTests, i.e. this should be src/Xamarin.Android.Build.Tasks/Test/Xamarin.Android.Build.Tests/BuildTest.cs.

[jonpryor]

I wonder why this file is indented with tabs while the other .csproj files are indented with spaces, two spaces per indent, e.g. Mono.Android.csproj.

Should it be fixed for consistency?

[jonpryor]

This also needs a way to run the unit tests, e.g. a make run-all-tests target, a'la Java.Interop.

[dellis1972]

@jonpryor I've updated this to move the UnitTests to Tests and added a runner.

[jonpryor]

This should be run-nunit-tests, to differentiate from the run-apk-tests in PR #32.

[jonpryor]

Sorry; this should likewise be NUNIT_TESTS.