cfs/test: Fix ro /sysroot problem

The `bootc internals cfs` test was running into an issue where pulling
an image into a composefs repository at `/sysroot/composefs` was failing
due to /sysroot being mounted ro. We did remount it rw in an earlier
cmd, but that was in a separate mount ns which did not carry over to the
next commands

Instead of globally remounting /sysroot as rw, we now simply create a
new repository at /var/tmp/sysroot/composefs and perform all operations
there.

Also, pass in `--insecure` to `bootc internals cfs ...` as in
the ostree case we're testing in xfs which does not support fs-verity

Signed-off-by: Pragyan Poudyal <pragyanpoudyal41999@gmail.com>

Author: Johan-Liebert1

tmt/tests/booted/readonly/030-test-composefs.nu

@@ -39,8 +39,12 @@ if $is_composefs {
     # When not on composefs, run the full test including initialization
     bootc internals test-composefs
     bootc internals cfs --help
-    bootc internals cfs oci pull docker://busybox busybox
-    test -L /sysroot/composefs/streams/refs/busybox
+
+    # We use a separate `/sysroot` as we need rw access to the repo which
+    # we can't get from `bootc internals cfs ...`
+    mkdir /var/tmp/sysroot/composefs
+    bootc internals cfs --insecure --repo /var/tmp/sysroot/composefs oci pull docker://busybox busybox
+    test -L /var/tmp/sysroot/composefs/streams/refs/oci/busybox
 }
 
 tap ok