test: close coverage gaps from the preceding batch

Untested paths surfaced while reviewing the preceding feature/fix commits:
- calendar-sync: a test that `-L' precedes `-l' in the worker command (separate `member' checks wouldn't catch a swap), plus a `:slow' tag on the real-subprocess worker test so it stays out of the default `make test' run.
- org-capture cache: a killed marker buffer invalidates the entry and the next resolution rescans without erroring on the stale marker, `cj/org-capture-clear-target-cache' actually empties the hash, and non-`file+headline' targets (`file', `file+olp', `file+function') fall through to the original `org-capture-set-target-location'.
- lorem-optimum: `cj/lipsum-title' on an empty chain returns "", not an error.
- calibredb-epub: a negative `cj/nov-margin-percent' is clamped up to 0 (text takes the full window width).
- mu4e attachments: the default save directory comes from the part's `:target-dir' and falls back to `~/Downloads/', and asking for the attachment at point on a header line fails with a `user-error'.

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