Improve testing of sdk snapshot generation

Adds TestHelper to provide general test helper functionality for use by
any test.

Adds testSdkResult, composed with TestHelper that encapsulates the
result of processing and sdk {..} definition and provides specialized
support for testing the build rules.

Dedups the analysis of the sdk build rules, and improves it to extract
more information, and in different forms. That is represented by the
snapshotBuildInfo struct.

Adds a CheckSnapshot() method which checks the snapshot for a specified
sdk version. It takes a list of functions that can each perform a check
on a specific facet of the supplied snapshotBuildInfo.

Methods are provided for tests to use to check the following facets:
* Generated Android.bp contents.
* Copy rules
* Merge zip inputs

This approach makes it possible for each test to customize what is
being checked without either duplicating functionality, causing a
proliferation of specialized forms of the CheckSnapshot method for
different types of tests or adding arguments for every possible check
that any test would need which would lead to lots of churn to existing
tests when new arguments are added.

The main testing improvement is for CheckSnapshot() to actually try and
load the Android.bp that is generated. In order to do that it was
necessary to create a mock filesystem populated with information from
the build rules, i.e. the destination files from every Cp command as
well as the destination directory from every repackage zip command.

That helps detect a number of sources of errors:
* Failing to copy a file/directory that is mentioned in the generated
  Android.bp file.
* Invalid properties.
* Invalid format of the .bp file.
* Integrity issues within the .bp file.

Bug: 143678475
Test: m conscrypt-module-sdk
Change-Id: I4d3fe18f86698186d18e7e8b32d2e319183f7f0c