Add a --version argument that also outputs features

And use it in the tests to skip libarchive tests when the build
doesn't support it.

tests/t0006-libarchive.sh

@@ -19,6 +19,10 @@
 
 set -e
 
+if ! ostree --version | grep -q -e '\+libarchive'; then
+    exit 77
+fi
+
 echo "1..7"
 
 . $(dirname $0)/libtest.sh