Add more tests to accept_test.go

[twifkak]

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Add all the tests in cl/243547227 that are missing from accept_test.go. Maybe require putting some of those on the wrong side, as #240 is still open.

[twifkak]

Also, per https://github.com/ampproject/amppackager/pull/296#discussion_r285801253:

Add some tests including \\" (escaped backslash followed by unescaped quote) and \\\" (escaped backslash followed by escaped quote), to guard against tokenize being replaced by something simpler but wrong.

[twifkak]

And maybe https://github.com/ampproject/amppackager/pull/296#discussion_r285801652, if a falsifiable test can be created:

I think if !inQuotes, this should be a parse error, per the tchar definition. I guess returning empty list would make sense in this case. Add a test for this if possible; maybe something like application/signed-exchange;v=b3;x=foo\,bar.