test_linux.sh: Do not fail if called twice

The script is set to exit on error, so mkdir failing because
/tmp/cni-rootless already exists aborts the test run. Call 'mkdir -p' to
avoid the spurious error.

Signed-off-by: Phil Sutter <psutter@redhat.com>

test_linux.sh

@@ -39,5 +39,5 @@ for t in ${PKG}; do
 done
 
 # Run the pkg/ns tests as non root user
-mkdir /tmp/cni-rootless
+mkdir -p /tmp/cni-rootless
 (export XDG_RUNTIME_DIR=/tmp/cni-rootless; cd pkg/ns/; unshare -rmn go test)