Add tests for verify_overlaps script

Refactor verify_overlaps to make it testable and add tests for the
comparison. It does make one significant change in behavior which is to
read each of the files produced by a bootclasspath_fragment into a dict
before comparison, rather than reading and comparing them a row at a
time. That allows it to reuse the code to read a CSV into a dict.

Bug: 194063708
Test: atest --host verify_overlaps_test
      m out/soong/hiddenapi/hiddenapi-flags.csv
      - manually change files to cause difference in flags to check
        that it detects the differences.
Change-Id: Ib70ac87fe089fc25e3bef18f367d4939bfc0cb8d