Set default lastStartTag in TokenizerTester

[sideshowbarker]

This change causes TokenizerTester to set a default lastStartTag value for any test which has a initialStates value but no lastStartTag value.

Otherwise, without this change, the tokenizer is never actually run for such missing-lastStartTag tests — which results in us failing 5 tests from the html5lib-tests suite.

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The html5lib-tests cases which fail without this change are the following:

• https://github.com/html5lib/html5lib-tests/blob/master/tokenizer/domjs.test#L28 (“Raw NUL replacement”)
• https://github.com/html5lib/html5lib-tests/blob/master/tokenizer/domjs.test#L38 (“NUL in CDATA section”)
• https://github.com/html5lib/html5lib-tests/blob/master/tokenizer/domjs.test#L284 (“CDATA content”)
• https://github.com/html5lib/html5lib-tests/blob/master/tokenizer/domjs.test#L290 (“CDATA followed by HTML content“)
• https://github.com/html5lib/html5lib-tests/blob/master/tokenizer/domjs.test#L296 (“CDATA with extra bracket”)

[hsivonen]

I pushed this via the command line as 23a8b13439f65a4e0cb963075cef78661aed264e and now I don't know how to make the GitHub PR metadata say merged instead of closed.

Thanks and sorry about the bad metadata state.