- don't make assumptions on where programs live (i.e., /bin/sh, /bin/rm,
/bin/mv) are not universal
- dont invoke shell when unnecessary
- improve error-handling

Build the tests lazily, i.e., not before `meson test`. This helps with
build-times.

No need for identical files in mu4e / guile dirs, move to build-aux.

In message, don't re-fill when unneeded. Add tests.

Fixes #2802.

Somewhere along the way, the body-txt and body-html fields were merged
into a single body field. Later, commit 8eac3922215c ("guile: re-enable
the guile support") finally removed support for Field::Id::BodyHtml from
mu:c:get-field.

Unfortunately mu.scm and the documentation are still stuck in the past, so
update them. mu:body-txt is now a synonym for mu:body, and mu:body-html
always returns #f.

I wanted to add a mu:body test also for the rfc822.1 message, but there's
currently a bug where its body text is is duplicated (issue #2802), so the
test would fail.

It's stated to be US-ASCII with 7-bit encoding, but it contains the
UTF-8 character '—'.

Fixing the encoding very slightly changes the average size of messages
reported by mu:average, so adjust the new expected value.