src/vfs: rename idmapped-mounts.c file

src/vfs: rename idmapped-mounts.c file

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. So rename the source file from
idmapped-mounts.c to vfstest.c before we split it into multiple source
files in the next patches.

[Zorro: fix generic/689]

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