test: explicitly "add" file before some commit in test-filecache.py
`hg commit -A` will revert the `hg addremove` step if the commit fails. However
`hg rollback` currently does not.
We are about to improve internal consistency around transaction and dirstate and the behavior of `hg rollback` will align on the other behavior in the process.
Before doing so, we make sure the test is using a separate call to `hg add` to
avoid the test scenario to be affected by that future change.
note: the behavior change for `hg rollback` seems fine as it affect a niche
usecase and `hg rollback` usage have been strongly discouraged for a while.
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