test: ignore IAB capabilities in `test-execute`

libcap v2.33 introduces a new capability set called IAB[0] which is shown
in the output of `capsh --print` and interferes with the test checks. Let's
drop the IAB set from the output, for now, to mitigate this.

This could be (and probably should be) replaced in the future by the
newly introduced testing options[1][2] in libcap v2.32, namely:
    --has-p=xxx
    --has-i=xxx
    --has-a=xxx

but this needs to wait until the respective libcap version gets a wider
adoption. Until then, let's stick with the relatively ugly sed.

Fixes: #15046

[0] https://git.kernel.org/pub/scm/libs/libcap/libcap.git/commit/?id=943b011b5e53624eb9cab4e96c1985326e077cdd
[1] https://git.kernel.org/pub/scm/libs/libcap/libcap.git/commit/?id=588d0439cb6495b03f0ab9f213f0b6b339e7d4b7
[2] https://git.kernel.org/pub/scm/libs/libcap/libcap.git/commit/?id=e7709bbc1c4712f2ddfc6e6f42892928a8a03782

(cherry picked from commit e9cdcbed)