tests: Fix checks where we expect a command to fail - #6625

Good catch!

I was convinced that the pattern ! command with -e aborts when command fails. This is not the case (the result of false is the same as ! true but somehow this doesn't matter).

It doesn't matter because POSIX shells explicitly exempt some patterns from triggering set -e, to make set -e be useful. For example it would be unusable if if thing_that_fails; then ...; else ...; fi crashed out as soon as thing_that_fails fails, without ever getting to the body of the else.

! foo is one of those patterns, as documented in bash(1):

              -e      Exit  immediately  if  a pipeline (which may consist of a
                      single simple command), a list,  or  a  compound  command
                      (see  SHELL GRAMMAR above), exits with a non-zero status.
                      The shell does not exit if the command that fails is part
                      of the command list immediately following a while or  un‐
                      til  keyword,  part  of the test following the if or elif
                      reserved words, part of any command executed in a  &&  or
                      ||  list except the command following the final && or ||,
                      any command in a pipeline but the last, or  if  the  com‐
                      mand's  return value is being inverted with !.

(last part of the quote).

Just to troll us, dash(1) (Debian/Ubuntu /bin/sh) doesn't document that ! has this effect, but I don't know whether it's the shell behaviour or just the documentation that actually differs from bash here. But Flatpak's tests explicitly use bash rather than /bin/sh, so we're insulated from that anyway.

dash(1) does have a nicer way to describe what set -e does, which might help you to have an appropriate mental model: it describes it as making the shell exit if an "untested command" fails, and then goes on to say that various things like if and the LHS of && are considered to be "explicitly tested" for the purposes of set -e.