test: Factorize common integration test functions (#6540)

test: Factorize common integration test functions (#6540)

All test/TEST* but TEST-02-CRYPTSETUP share the same check_result_qemu()
and test_cleanup(), so move them into test_functions and only override
them in TEST-02-CRYPTSETUP.

Also provide a common test_run() which by default assumes that both QEMU
and nspawn tests are run. Particular tests which don't support either
need to explicitly opt out by setting $TEST_NO_{QEMU,NSPAWN}. Do it this
way around to avoid accidentally forgetting to opt in, and to encourage
test authors to at least always support nspawn.