The <dependenciesToScan> configuration attribute relies solely on installed artifacts. This is an issue when the targeted dependencies were built as part of the current session. The net result is that stale artifacts are used (i.e. if the dependency has changed since it was last installed) or the tests are not executed at all (if the dependency has not been previously installed.
Attached is a sample project that illustrates this issue:
Given I have a multi-module project
And the first module built includes test classes as part of the project artifact
And subsequent modules scan the first for unit tests to execute
When I execute the test goal (prior to any install)
Then the build should succeed
And tests should be executed with each module