Support use case when test cases are not located in test bench top level.

[kraigher]

In VUnit it is preferred to always let the test bench contain the test cases on the top level and have it instantiate any fixtures rather than the other way around. We believe all test bench / test fixture pairs can be structured in this way without any negative effects.

However there might be many legacy test benches / fixture pairs structured in the opposite way.
To accommodate these legacy structures we will add a test_bench.scan_tests_from(file_name) method to the Python-API.