test: Robustify cleanup in TestStorageNfs.testNfsClient
test: Robustify cleanup in TestStorageNfs.testNfsClient

When the test fails at a place while /mnt/test is mounted, removing the
mountpoint fails. Unmount it first.

mvollmer/cockpit,cockpit-project/cockpit,mvollmer/cockpit,martinpitt/cockpit,martinpitt/cockpit,martinpitt/cockpit,cockpit-project/cockpit,mvollmer/cockpit,cockpit-project/cockpit,mvollmer/cockpit,martinpitt/cockpit,martinpitt/cockpit,cockpit-project/cockpit,mvollmer/cockpit,cockpit-project/cockpit