add some missing tokio::fs function tests etc.

[brodycj]

Motivation

I tried experimenting with no-panic (long story) and discovered that a number of the functions in tokio/src/fs are not really tested. I was able to stick panic!() calls into over 30 of these functions without causing any test failures.

I hope to improve the test coverage as discussed further below.

Solution

I started by injecting lines of code like this into tokio/src/fs to help keep track of what is and is not tested:

assert!(std::env::var("XXX_KEEP_UNTESTED_XXX").is_ok());

to cause the untested tokio::fs functions to panic without triggering any build issues.

I have been able to add most of the missing test coverage in my own workspace. I started this PR with test coverage of tokio::fs::canonicalize() and have pushed all of the changes to start testing the previously untested tokio::fs functions.

other testing enhancements:

• improve testing & simplify test code in tokio/tests/fs_link.rs
• update WASI comments in fs tests

TODO items - done ✅

• finish the missing test coverage for tokio::fs
• ensure that all injected panicking code is removed
• revert 258278e
• ensure that there are no remaining XXX todo items

---

Future work ideas time permitting beyond this PR - updated:

• improve test coverage in some other places such as tokio::io, tokio::net, etc.
• explore & propose alternative ideas for code coverage - I already found cargo-llvm-cov with permissive licensing and design to work with faster nexte.st for testing
• look into mutation testing
• investigate if there could be any way to get some of the fs operations working together with WASI / WASM
• review source & generated documentation for tokio::fs & Tokio in general

[brodycj]

I noticed that this WASI comment that I copied was sometimes "file operations" and other times "directory operations". My apologies for possibly veering off-topic but from a little searching it looks to me like some fs operations may be possible with WASI but not all and WASI directory access would not work very easily with Tokio. I went ahead and used sed to apply a slightly more general statement across tokio/tests/fs_*.rs.

[Darksonn]

Thank you!