Rollup merge of #150524 - test-build-std, r=jieyouxu,kobzol

Test that -Zbuild-std=core works on a variety of profiles

See [#t-infra > Non-blocking testing for -Zbuild-std?](https://rust-lang.zulipchat.com/#narrow/channel/242791-t-infra/topic/Non-blocking.20testing.20for.20-Zbuild-std.3F/with/565837190) for some background.

This is an incredibly CPU-hungry run-make-cargo test, but at least on my desktop the entire suite only takes a minute.

Author: GuillaumeGomez

src/tests/compiletest.md

@@ -78,6 +78,10 @@ The following test suites are available, with links for more information:
 
 [`run-make`](#run-make-tests) are general purpose tests using Rust programs.
 
+### The build-std test suite
+
+[`build-std`](#build-std-tests) test that -Zbuild-std works.
+
 ### Rustdoc test suites
 
 | Test suite                           | Purpose                                                                  |
@@ -429,6 +433,12 @@ use cases that require testing in-tree `cargo` in conjunction with in-tree `rust
 The `run-make` test suite does not have access to in-tree `cargo` (so it can be the
 faster-to-iterate test suite).
 
+### `build-std` tests
+
+The tests in [`tests/build-std`] check that `-Zbuild-std` works. This is currently
+just a run-make test suite with a single recipe. The recipe generates test cases
+and runs them in parallel.
+
 #### Using Rust recipes
 
 Each test should be in a separate directory with a `rmake.rs` Rust program,