tests: Simplify running cmp_out more than once

tests: Simplify running cmp_out more than once

When cmp_out detects a test failure, it returns non-zero status, which
terminates the test script unless caught.

Accumulate failures in a global variable checked at exit instead, so
running it multiple times in a test script just works.

Signed-off-by: Markus Armbruster <armbru@pond.sub.org>