test-jar misses sources from "generate-test-sources" phase

When the goal source:test-jar is run directly from the command-line, the plugin will miss source files that get generated during the "generate-test-sources" phase as the mojo currently only requires the "generate-sources" phase.

Though simply increasing the mojos lifecycle requirement would solve the problem, it kind of looks ugly to have the lengthy and potentially failing "compile" phase get run just to package test sources (first time I realize practical limitations of Maven's simple waterfall lifecycle compared to partially ordered goals...). An additional mojo like "generated-test-jar" might therefore be the best solution to keep the majority of use-cases fast but support the more sophisticated cases as well.