The tapestry-test module is meant to be "just a couple of base classes to make it easier to build integration test suites around Selenium.".
However, org.apache.tapestry5.internal.test.PageTesterContext for tapestry-core uses TapestryTestConstants from tapestry-test. So it seems that anyone doing any kind of tests requires the tapestry-test module and so also imports all the selenium dependencies.
That dependency problem could be resolved by moving TapestryTestConstants into tapestry-core.