If your job uses the JUnit Realtime Test Reporter Plugin, you can display the progress bar while the project is building.
There are actually 2 progress bars stacked together. One based on the number of tests and the second based on the test execution time. A tooltip will display the tests count and the estimated remaining time. All the stats are based on the result from the previous build and can change a lot from build to build (multiple concurrent jobs can slow down the test execution).