checkout: also chmod in the user checkout case

When falling back to copying, we previously would only chmod checked out
files in the non-user-checkout mode. Fix this by always doing chmod.
The file_mode was being prepared but never actually applied.

Add a basic test in the archive-z2 --> usermode checkout case in which
we're guaranteed to always fall back to copy mode.

Closes: #633

Closes: #903
Approved by: cgwalters

tests/libtest-core.sh

@@ -110,6 +110,13 @@ assert_file_has_content_literal () {
     fi
 }
 
+assert_file_has_mode () {
+    mode=$(stat -c '%a' $1)
+    if [ "$mode" != "$2" ]; then
+        fatal "File '$1' has wrong mode: expected $2, but got $mode"
+    fi
+}
+
 assert_symlink_has_content () {
     if ! test -L "$1"; then
         fatal "File '$1' is not a symbolic link"