Data used in tests is called test data and is located in this folder, or sub-folders thereof. This is not for test files, this folder should not contain test code, only data used in tests.
The test wrapper testWithGolden('<name>', (ctx) async { will register a test case, and create a file: test/testdata/goldens/path/to/myfile_test/<name>.txt , where path/to/myfile_test.dart is the name of the file containing the test case, and <name> is the name of the test case.
Any calls to ctx.run will run pub and compare the output to a section in the golden file. If the file does not exist, it is created and the test is marked as skipped. Thus, it is safe to delete all files in test/testdata/goldens and recreate them -- just carefully review the changes before committing.
Maintaining goldens:
test/testdata/goldens/.test/testdata/goldens/.git diff test/testdata/goldens/.