SkPDF/Tests: imporve test coverage.

Also: make sure that all SkPDF unit tests are named SkPDF_* to
make testing changes to SkPDF easier.  Other cleanup.

Add test: SkPDF_pdfa_document to verify that flag in public API
works.

SkPDF_JpegIdentification test: test slightly malformed JPEGs to
verify that all code paths work.

BUG=skia:
GOLD_TRYBOT_URL= https://gold.skia.org/search?issue=2322133003

Review-Url: https://codereview.chromium.org/2322133003
6 files changed