write_source_file's tests should check the executable bit

Whether the output file is executable is part of write_source_file's declared state, so it should be checked in tests along with diffing the contents. Currently, you can run the updater, manually flip the executable bit on the output, and the tests will still pass.

Search for write_source_file and its existing tests, then run the relevant test to establish the current behavior. Update the test coverage so it checks both the generated contents and whether the output file has the declared executable bit; done means a manually changed executable bit no longer passes unnoticed.

Written by the indexing model from the issue text.