Add VCN tests to postsubmit

This patch is to fulfil the prerequisite for adding these
tests to presubmit in this TEST_MAPPING file, as per
go/test-mapping-slo-guide. It also adds postsubmit coverage
for future VCN changes.

Bug: 369703242
Test: build
Flag: EXEMPT TEST_MAPPING change
Change-Id: I83c9034b56e7a6de458f26b8a4f01df08f355e87