Ignore non-zero return code in make (at)shell

We want these to always be 0 so that cmake doesn't print an "error" when
leaving the shell (if the last command happened to fail).

However, since it's a cmake thing, handle that in cmake instead of
mktree backends.

No functional change, just cosmetic.

tests/CMakeLists.txt

@@ -111,12 +111,12 @@ configure_file(atlocal.in atlocal @ONLY)
 configure_file(package.m4.in package.m4 @ONLY)
 
 add_custom_target(atshell
-	COMMAND ./mktree atshell
+	COMMAND ./mktree atshell ||:
 	DEPENDS tree
 )
 
 add_custom_target(shell
-	COMMAND ./mktree shell
+	COMMAND ./mktree shell ||:
 	DEPENDS tree
 )
 

tests/mktree.fedora

@@ -177,12 +177,10 @@ case $CMD in
 
         echo "$ATSHELL_MOTD"
         $SHELL
-        exit 0
     ;;
     shell)
         mount_tree $SANDBOX_DIR
         snapshot shell "$@"
-        exit 0
     ;;
     check)
         mount_tree

tests/mktree.podman

@@ -32,7 +32,6 @@ case $CMD in
         fi
         $PODMAN run --privileged -it --name $NAME --hostname sandbox \
                     -e ROOTLESS=$ROOTLESS -v $PWD:/srv:z $IMAGE "$@"
-        exit 0
     ;;
     check)
         # Pass select options to $PODMAN