Complete unit tests for ServerX and ClientX records in tcpinfo and pt parsers

[stephen-soltesz]

This change completes the unit tests omitted from https://github.com/m-lab/etl/pull/1004

This change moves the fakeAnnotator type from the tcpinfo_test.go file to the common parser_test.go for use by multiple parsers.

This change adds ServerX and ClientX value tests to the tcpinfo parser. This change adds ServerX and ClientX value tests to the pt parser for .paris and .jsonl file types.

Because correct annotations require well formatted archive URLs, additional changes ensure that valid URLs are provided to unit tests.

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This change is 

[coveralls]

Pull Request Test Coverage Report for Build 6544

• 6 of 6 (100.0%) changed or added relevant lines in 1 file are covered.
• 3 unchanged lines in 1 file lost coverage.
• Overall coverage increased (+0.09%) to 62.861%

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Files with Coverage Reduction	New Missed Lines	%
active/poller.go	3	67.86%
<!--	Total:	3		-->

Totals
Change from base Build 6540:	0.09%
Covered Lines:	3507
Relevant Lines:	5579

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💛 - Coveralls

[stephen-soltesz]

Thank you!