vfstest: rename struct t_idmapped_mounts

vfstest: rename struct t_idmapped_mounts

The idmapped mounts test suite has grown to cover a lot of generic vfs
functionality that is not concerned with idmapped mounts at all.

As was discussed upstream it's time to rename it to something that
reflects its generic nature.

Rename the basic structure used for the tests from struct
t_idmapped_mounts to struct test_struct.

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