utils: add struct vfstest_info

utils: add struct vfstest_info

So far we were able to rely on global variables as the idmapped mount
test suite was a single source file. But that gets messy when we split
it into multiple source files in the following patches. Instead of
global variables introduce struct vfstest_info which provides all
information about the test suite run and pass it to all tests.

Acked-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Christian Brauner (Microsoft) <brauner@kernel.org>
Signed-off-by: Zorro Lang <zlang@kernel.org>