tests: workaround for shells with broken 'set -e'

tests: workaround for shells with broken 'set -e'

* tests/dist-formats.tap: Some versions of the BSD shell wrongly
bail out when the 'errexit' shell flag is active and the left-hand
command in a "&&" list fails and that list is the *last* command
of a "case" statement.  This was causing an incorrect initialization
of the '$missing_compressors' variable, and thus potential spurious
failures when non-existing programs were assumed to be available.