add tests for autofix files, fix 91x autofix, and rejig test files to…

… accomodate the new tests

CONTRIBUTING.md

@@ -32,8 +32,10 @@ To check that all codes are tested and documented there's a test that error code
 ## Test generator
 Tests are automatically generated for files in the `tests/eval_files/` directory, with the code that it's testing interpreted from the file name. The file extension is split off, if there's a match for for `_py\d*` it strips that off and uses it to determine if there's a minimum python version for which the test should only run.
 
-### autofix files
-Checks that have autofixing can have a file in the `tests/autofix_files` directory matching the filename in `tests/eval_files`. The result of running the checker on the eval file with autofix enabled will then be compared to the content of the autofix file and will print a diff (if `-s` is on) and assert that the content is the same. `--generate-autofix` is added as a pytest flag to ease development, which will print a diff (with `-s`) and overwrite the content of the autofix file. Also see the magic line marker `pass # AUTOFIX_LINE ` below
+### `# AUTOFIX`
+Files in `tests/eval_files` with this marker will have two files in `tests/autofix_files/`. One with the same name containing the code after being autofixed, and a diff file between those two.  
+During tests the result of running the checker on the eval file with autofix enabled will be compared to the content of the autofix file and will print a diff (if `-s` is on) and assert that the content is the same. `--generate-autofix` is added as a pytest flag to ease development, which will print a diff (with `-s`) and overwrite the content of the autofix file.  
+Files without this marker will be checked that they *don't* modify the file content.
 
 ### `error:`
 Lines containing `error:` are parsed as expecting an error of the code matching the file name, with everything on the line after the colon `eval`'d and passed as arguments to `flake8_trio.Error_codes[<error_code>].str_format`. The `globals` argument to `eval` contains a `lineno` variable assigned the current line number, and the `flake8_trio.Statement` namedtuple. The first element after `error:` *must* be an integer containing the column where the error on that line originates.