Properly check the deapexed files in tests.

The tests in bootclasspath_fragment_test setup the environment to have
two APEXes: a source one and a prebuilt one. Before this change, the
tests for prebuilt incorrectly check the contents in the source one.
This change introduces a new function that properly checks files
deapexed from the prebuilt one.

Bug: 241823638
Test: m nothing
Change-Id: I1865c20b198d50e7ebc8ebfb9f7c71394a225ab7