Minimize convert_integer_literal tests

[matklad]

There are a lot of tests for this assist: https://github.com/rust-analyzer/rust-analyzer/blob/060c8b2c96a0de4a131c4d780d2aac80afe13de8/crates/assists/src/handlers/convert_integer_literal.rs

I think the test look like MxN matrix, and mostly hit the same code paths over and other again. We should minimize the test suite to cover all interesting paths with as few tests as possible.

And advanced way to find redundant tests is by setting up coverage (haven't done this before for rust though). But I guess in this case, we can just eyeball duplication.

See also https://github.com/rust-analyzer/rust-analyzer/blob/060c8b2c96a0de4a131c4d780d2aac80afe13de8/docs/dev/style.md#minimal-tests

[m-ysk]

@matklad Hello! Can I work on this issue?

[lnicola]

@m-ysk it's yours.