Move libXtest into libX/tests

[ghost]

This change moves:

1. libcoretest into libcore/tests
2. libcollectionstest into libcollections/tests

This is a follow-up to #39561.

r? @alexcrichton

[ghost]

Tidy failed with:

/home/stjepan/work/rust/src/libcore/tests/num/flt2dec/strategy/dragon.rs:49: platform-specific cfg: cfg!(target_env = "msvc")
/home/stjepan/work/rust/src/libcore/tests/num/flt2dec/estimator.rs:14: platform-specific cfg: cfg(not(target_os = "emscripten"))
thread 'main' panicked at 'some tidy checks failed', src/tools/tidy/src/main.rs:63
note: Some details are omitted, run with `RUST_BACKTRACE=full` for a verbose backtrace.

I'm not sure what's up with that... Any ideas?

[petrochenkov]

@stjepang
Tidy checks that crates do not contain platform specific cfgs unless whitelisted.
You should probably search for libcoretest in tidy whitelists and whitelist it under the new location.

[alexcrichton]

@borsd: delegate=stjepang

r=me as soon as tidy is fixed

[bors]

✌️ @stjepang can now approve this pull request

[malbarbo]

@stjepang What do you think about striping the lib prefix in the projects directory names? This looks like an ancient convention...

[ghost]

@malbarbo Sounds like a good idea to me. I'm all for removing vestigial conventions that don't make sense at this time anymore. However, I wonder if there are traps to watch out for when doing such a massive rename operation.

Here's a list of the 54 subdirectories that start with 'lib': list
Do we want to strip 'lib' from all of them? Should we leave some of those alone?

Another thing that is slightly odd is that directories like libcore and libcollections lump *.rs files together with Cargo.toml. Do we want to follow conventions established by Cargo and move all those *.rs files into src subdirectory?

Is there any other "housekeeping" work that needs to be done?

@alexcrichton What are your thoughts on this?

[alexcrichton]

Let's not rename things just yet, that's a much broader discussion. Let's stick to this PR just moving the tests to the appropriate locations.

[malbarbo]

@stjepang I would like to see all vestigial conventions vanish. You raised some interesting questions, what about open an specific issue to address these questions?

@alexcrichton You are right, that's a broader discussion. I think @stjepang (like me) was just asking an opinion, the intent was not to change this PR, sorry for being unclear.

[ghost]

@bors r=alexcrichton

[bors]

📌 Commit 13c744f has been approved by stjepang

[bors]

💡 This pull request was already approved, no need to approve it again.

• There's another pull request that is currently being tested, blocking this pull request: Switch to vault.centos.org #41045

[bors]

📌 Commit 13c744f has been approved by stjepang

[ghost]

@bors r=alexcrichton

[bors]

💡 This pull request was already approved, no need to approve it again.

• There's another pull request that is currently being tested, blocking this pull request: Switch to vault.centos.org #41045

[bors]

📌 Commit 13c744f has been approved by alexcrichton