fix: test: Reorganize imports in tests_synthrecord.py

bin/tests/system/synthrecord/tests_synthrecord.py imports hypothesis
before importing isctest.hypothesis, which causes the "synthrecord"
system test to fail on platforms on which the Hypothesis module is not
available.  Reorganize Python imports in tests_synthrecord.py to fix the
above issue and also to make it more in line with other similar test
scripts.

Closes #1586

Merge branch '1586-reorganize-imports-in-tests_synthrecord.py' into 'main'

See merge request isc-projects/bind9!11046