wcfs: tests: Test WCFS CLI
https://lab.nexedi.com/nexedi/wendelin.core/-/merge_requests/38 attempts to fix bugs introduced with https://lab.nexedi.com/nexedi/wendelin.core/-/commit/e51bef0d26ebaa32d81f7a92370d3f3f86521e6a. We didn't see these bugs in test results, because the WCFS CLI codepath is not covered in our tests. This patch adds coverage of this codepath to increase the likelihood that issues are quickly detected.
/cc @kirr