_stbt.core._tesseract: Refactor: Remove argument default values
_stbt.core._tesseract: Refactor: Remove argument default values

So that we don't repeat the default values in multiple places (it can be
misleading especially for arguments that all callers always provide
explicitly). In the next commit I'll be adding an argument whose default
value can't be `None`.

stb-tester/stb-tester,stb-tester/stb-tester,stb-tester/stb-tester,stb-tester/stb-tester