test-manifest: make manifesttest a base class that is extended
The implementation of the testmanifest test case is moved to a new base class,
which is then extended to make the testmanifest. And instead of testmanifest,
the subclass is named testmanifestdict because, well, that's what it's testing.
This refactoring makes it possible to create alternate versions of what was
formerly testmanifest, improving test coverage of different manifestdict
implementations.
<Include>
<?if $(var.Platform) = "x64" ?>
<?define IsX64 = yes ?>
<?else?>
<?define IsX64 = no ?>
<?endif?>
</Include>