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
diff --git a/tests/PDFInvalidBitmapTest.cpp b/tests/PDFInvalidBitmapTest.cpp
index c9685f7..10292bd 100644
--- a/tests/PDFInvalidBitmapTest.cpp
+++ b/tests/PDFInvalidBitmapTest.cpp
@@ -43,9 +43,12 @@
 
 }  // namespace
 
-DEF_TEST(PDFInvalidBitmap, reporter) {
+DEF_TEST(SkPDF_InvalidBitmap, reporter) {
     SkDynamicMemoryWStream stream;
     sk_sp<SkDocument> document(SkDocument::MakePDF(&stream));
+    if (!document) {
+        return;
+    }
     SkCanvas* canvas = document->beginPage(100, 100);
 
     canvas->drawBitmap(SkBitmap(), 0, 0);