You can accomplish this by the Hide reference option for referenced libraries. This option lets you hide TcUnit from your other projects.
Let's assume you've developed a library MyLibrary, which has tests written in TcUnit. You make a PLC project MyProject, which references MyLibrary.
If you use Hide reference on TcUnit in MyLibrary, then TcUnit won't show up in the imports list of MyProject. You can find it in the Properties tab: