coreos-base/oem-gce: Shrink the sysext image by removing python tests

Not only python is being installed, but also some extra packages like
setuptools. These amount of tests seems to be growing, which is not a
bad thing per se, but we really don't need them in the GCE OEM sysext.

sdk_container/src/third_party/coreos-overlay/coreos-base/oem-gce/files/manglefs.sh

@@ -0,0 +1,15 @@
+#!/bin/bash
+
+set -euo pipefail
+
+rootfs=${1}
+
+# Remove test stuff from python - it's quite large.
+for p in "${rootfs}"/usr/lib/python*; do
+    if [[ ! -d ${p} ]]; then
+        continue
+    fi
+    # find directories named tests or test and remove them (-prune
+    # avoids searching below those directories)
+    find "${p}" \( -name tests -o -name test \) -type d -prune -exec rm -rf '{}' '+'
+done