Drop unit and system tests on amd64cross32

Drop unit and system tests on amd64cross32

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:

- #3269 - doth-ns1 frequently failed to start
- #3316 - quota_test sometimes fails to create new thread
- #3956 - tcp-ns1 sometimes fails to allocate memory

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 f86d4f3bd8c5755dbc6d115e4a274e74c3bd856b)