Make the main cgitrc filter check run the filter

The check asked for the summary page, which never runs an about filter,
so it passed with exec:/bin/true on macOS, which has no such file, and
with a filter that does not exist at all. Its first line read the
stderr the previous test left behind. execvp searches the PATH, so a
plain true runs the same on macOS, Debian and busybox.

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