Grow TEST-94-COCO from a single inline check into the reusable structure
that can be used by multiple future subtests:
- lib.sh: code shared between subtests on the host side.
- guest-test.sh: the testcase_coco_* checks.
- guest-test-runner.sh: runs each testcase_coco_* check, ships guest
side testcase results over vsock.
Helpers live under test/units/TEST-94-COCO/. The vsock records are
best-effort detail, the aggregate exit status stays the authoritative
pass/fail.
Split the test into subtests per platform/boot flow.
Signed-off-by: Paul Meyer <katexochen0@gmail.com>