lintian-junit-report: Support for marking tests skipped

This change brings a possibility to mark selected lintian checks as
skipped (in terms of JUnit report). Skipped test cases are considered
less than passed by Jenkins and marked on test result trend, but the
default JUnit results publisher does not set job result to UNSTABLE when
seeing them. Thanks to that you can ignore selected results but still
have them available in the report. The behaviour is controlled by two
new command line options. Example:

lintian-junit-report -w --mark-warnings-skipped \
	--mark-as-skipped=no-debian-copyright,no-copyright-file [...]

Last option takes a list of lintian tags to mark skipped.