Make test output more readable

[TorstenStueber]

Currently the test runner uses console.log statements directly. Instead all output should be collected in memory and then be displayed as required.

This allows to:

• display different verbosity levels (to be defined later)
• display different nesting levels of nested calls (as done by the forge test runner)
• not to display the different test runs of fuzz tests
• only show the output of failed tests

For now it is enough that the test runner then displays the output from memory the same way as it is shown now. We will define the different output scenarios later.