support/testing: update to the new text representation of capabilities

Since libcap 2.41, the text representation of capabilities now use
the '=' instead of '+' [1].

This break our capabilities tests since we still use the old
representation.

[1] https://git.kernel.org/pub/scm/linux/kernel/git/morgan/libcap.git/commit/?id=177cd418031b1acfcf73fe3b1af9f3279828681c

Fixes:
https://gitlab.com/buildroot.org/buildroot/-/jobs/830981737



Signed-off-by: Romain Naour <romain.naour@gmail.com>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>