test: adapt to the new capsh format

Since libcap v2.29 the format of cap_to_text() has been changed which
makes certain `test-execute` subtest fail. Let's remove the offending
part of the output (dropped capabilities) to make it compatible with
both the old and the new libcap.

(cherry picked from commit 9569e385)
(cherry picked from commit 1325dfb5)