On Eclipse, create the tests to be shared on one of the projects, as usual.
When you are happy with the test result, go to the other folder and create a new Folder inside:
- Right Button on Project >> New >> Folder
- Press "Advanced Button"
- Select "Link to alternate location (Linked Folder)
- Press the "Browse..." button and browse to the folder where previous tests are located.
Finally, right click on the linked folder and select:
- Build Path >> Use as Source folder.