Add --test-threads option to test binaries

[jupp0r]

This change allows parallelism of test runs to be specified by a
command line flag named --test-threads in addition to the existing
environment variable RUST_TEST_THREADS. Fixes #25636.

[rust-highfive]

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[alexcrichton]

Seems reasonable to me, thanks for the PR! I'd also be fine just calling this --threads.

cc @rust-lang/tools

[jupp0r]

I thought about calling the flag --threads, but decided against it because other threads might as well be spawned in tests and I wanted to avoid the impression that the flag limits the overall number of threads being created. I don't have a strong opinion, though.

[brson]

sgtm

[alexcrichton]

Discussed with the tools team --test-threads seemed like a good choice to us, thanks @jupp0r!

@bors: r+

[bors]

📌 Commit 6ca9094 has been approved by alexcrichton

[bors]

⌛ Testing commit 6ca9094 with merge 2b7ea14...

[bors]

☀️ Test successful - auto-linux-32-nopt-t, auto-linux-32-opt, auto-linux-64-cargotest, auto-linux-64-cross-freebsd, auto-linux-64-debug-opt, auto-linux-64-nopt-t, auto-linux-64-opt, auto-linux-64-opt-no-mir, auto-linux-64-opt-rustbuild, auto-linux-64-x-android-t, auto-linux-cross-opt, auto-linux-musl-64-opt, auto-mac-32-opt, auto-mac-64-opt, auto-mac-64-opt-rustbuild, auto-mac-cross-ios-opt, auto-win-gnu-32-opt, auto-win-gnu-32-opt-rustbuild, auto-win-gnu-64-opt, auto-win-msvc-32-opt, auto-win-msvc-64-cargotest, auto-win-msvc-64-opt, auto-win-msvc-64-opt-no-mir, auto-win-msvc-64-opt-rustbuild
Approved by: alexcrichton
Pushing 2b7ea14 to master...

[ghost]

Is it possible to check/assert for the number of threads in a test-case ?
Some of my tests shall run in a single thread. This flags solves half of my problem, but need to assert/ensure that, no other test-threads effects my running test if the command line argument was forgotten.