Allow assert passed tests expanded results

[tisonkun]

From the documents, it seems current pass tests only assert that "compile successfully and have a main function that does not panic when the compiled binary is executed".

Often, it would be helpful when a proc-macro author wants to assert the expanded result is as expected.

Does try_build provide such ability? Or I should write an extra script to run cargo expand and diff the output manually.

[ptagl]

I'm looking for such a feature as well. I would like a test that explicitly tests that the expansion of a Rust file matches the expected output.

If I find time to work on it, I'll try to contribute with a PR, but need to review and understand the existing code first.