Use realstd current thread static variables in tests

[sardok]

Fixes #150053. This PR basically exports thread::current::CURRENT & thread::current::ID* static variables and be referred to them when testing. I also added a minor refactoring which collects symbols referred as readlstd::... in test_internals module.

If approved, i would kindly ask this PR to be beta nominated.

Thank you.

[rustbot]

r? @Mark-Simulacrum

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[rust-log-analyzer]

The job aarch64-gnu-llvm-20-1 failed! Check out the build log: (web) (plain enhanced) (plain)

Click to see the possible cause of the failure (guessed by this bot)

test io::buffered::tests::line_write_all ... ok
test io::buffered::tests::line_write_all_error ... ok
test io::buffered::tests::long_line_flushed ... ok
panicked at library/core/src/ffi/c_str.rs:408:17:
assertion failed: !bytes.is_empty() && bytes[bytes.len() - 1] == 0
thread panicked while processing panic. aborting.
error: test failed, to rerun pass `-p std --lib`

Caused by:
  process didn't exit successfully: `/checkout/obj/build/aarch64-unknown-linux-gnu/stage2-std/aarch64-unknown-linux-gnu/release/deps/std-7a2bc8683455cfc5 -Z unstable-options --format json` (signal: 6, SIGABRT: process abort signal)
Bootstrap failed while executing `--stage 2 test --skip compiler --skip src`
Build completed unsuccessfully in 0:41:00
  local time: Thu Dec 18 16:06:53 UTC 2025
  network time: Thu, 18 Dec 2025 16:06:53 GMT
##[error]Process completed with exit code 1.