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/PDFGlyphsToUnicodeTest.cpp b/tests/PDFGlyphsToUnicodeTest.cpp
index 3ba8870..f65ee3e 100644
--- a/tests/PDFGlyphsToUnicodeTest.cpp
+++ b/tests/PDFGlyphsToUnicodeTest.cpp
@@ -25,7 +25,7 @@
     return memcmp(data->bytes() + offset, buffer, len) == 0;
 }
 
-DEF_TEST(ToUnicode, reporter) {
+DEF_TEST(SkPDF_ToUnicode, reporter) {
     SkTDArray<SkUnichar> glyphToUnicode;
     SkTDArray<uint16_t> glyphsInSubset;
     SkBitSet subset(kMaximumGlyphCount);