Add --read-only argument to mount_tree

Make it more obvious when the tree is mounted as read-only.
No functional change.

tests/mktree.fedora

@@ -70,10 +70,14 @@ dnf()
 
 mount_tree()
 {
+    local dir=$SANDBOX_DIR
+    if [ "$1" == "--read-only" ]; then
+        unset dir
+    fi
     source ./atlocal
     RPMTREE=$INST_DIR:$BASE_DIR
-    RPMTEST=${1:-$SANDBOX_DIR}/tree
-    snapshot mount $1
+    RPMTEST=$SANDBOX_DIR/tree
+    snapshot mount $dir
 }
 
 clean_up()
@@ -170,7 +174,7 @@ case $CMD in
     ;;
     atshell)
         set -a
-        mount_tree $SANDBOX_DIR
+        mount_tree
 
         export CACHE_DIR
         export BASE_DIR=$RPMTEST
@@ -180,11 +184,11 @@ case $CMD in
         $SHELL
     ;;
     shell)
-        mount_tree $SANDBOX_DIR
+        mount_tree
         snapshot shell "$@"
     ;;
     check)
-        mount_tree
+        mount_tree --read-only
         snapshot exec --tmpfs /tmp --bind $PWD $PWD --setenv RPMTREE $RPMTREE \
                       sh -c 'cd '$PWD' && exec ./rpmtests "$@"' rpmtests "$@"
     ;;