Cleanup SBOM test script for later adding tests of generating SBOM of

unbundled APKs/APEXs.

Bug: 266726655
Test: build/soong/tests/sbom_test.sh
(cherry picked from https://android-review.googlesource.com/q/commit:cd9948f66e9686e83cbe86d2b8ee998564caff0f)
(cherry picked from https://googleplex-android-review.googlesource.com/q/commit:1b2badc435c135e3e2c5996f4a5a352da1bb179a)
Merged-In: I2bb3e8405dad716837d3004d4c6fd54ae80527c1
Change-Id: I2bb3e8405dad716837d3004d4c6fd54ae80527c1