[9.18] Drop unit and system tests on amd64cross32

Merged Michal Nowak requested to merge mnowak/drop-unit-and-system-test-jobs-9.18 into bind-9.18

amd64cross32 jobs run a cross-compiled 32-bit BIND 9 on the 64-bit platform. This approach brings no build issues, and it is a good way to catch 32-bit issues in BIND 9 code-base and ensure cross-compilation keeps working.

Unfortunately, running unit and system tests is a problem:

There's little value in running unit and system tests on the amd64cross32 platform, and it only means the effort spent by QA by looking into what turns out to be 32-bit limitations.

(cherry picked from commit f86d4f3b)

Backport of MR !7717 (merged).

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