tests/vm/README: fix documentation path in tests/vm/README

tests/vm/README: fix documentation path in tests/vm/README

The README file in tests/vm/ points to a non-existent file,
docs/devel/testing.rst. Update the README to point to
docs/devel/testing/main.rst, which now contains information
about VM testing.

Signed-off-by: Haseung Bong <hasueng@gmail.com>
Fixes: ff41da50308 ("docs/devel: Split testing docs from the build docs and  move to separate folder")
Reviewed-by: Thomas Huth <thuth@redhat.com>
Message-ID: <20250607060456.28902-1-hasueng@gmail.com>
Signed-off-by: Thomas Huth <thuth@redhat.com>