tests/frontend: Refactor tests for default targets

tests/frontend/buildcheckout.py is already very crowded with all sorts of tests. Since this test is not really about testing bst build but rather about testing whether BuildStream can detect the default targets correctly, move it to a separate test module.

At the same time, also improve the actual tests. Currently the test compares the number of sub-directories in BuildStream log directory with the number of elements. Change it such that we directly query BuildStream about what's been built rather than trying to guess.

While doing so, sanitize its project directory to make it clearer what's being tested.