Test system malloc overriding when using dylibs

Test the Rust crates mimalloc and tikv-jemallocator and their ability to override the system allocator.

Check out the GitHub Actions runs for details on the failures. Currently macOS works, I am unsure about Linux.

See also:

• My jemallocator PR.
• My mimalloc PR.

This only tests when Rust invokes the linker - this works differently when that isn't the case, since then Rust won't get a chance to insert it's symbols.o trick (which is what this repository tests).

Test unused

Test that the allocator is overidden, even when the main binary doesn't allocate.

This is important in case allocator symbols are looked up using dlsym.

cargo run -p test-unused --features mimalloc
cargo run -p test-unused --features jemalloc

Test dylib

Test that the allocator in linked dynamic libraries are also overidden.

This is important to ensure that when calling into a dynamic library that e.g. returns a pointer that it guarantees is allocated with libc::malloc, we can free it with libc::free.

cargo run -p test-dylib --features mimalloc
cargo run -p test-dylib --features jemalloc

cargo run -p test-dylib --features dylib-mimalloc,dylib-mentions
cargo run -p test-dylib --features dylib-jemalloc,dylib-mentions # Works on current, but doesn't override the allocator

cargo run -p test-dylib --features dylib-mimalloc # Works on current, but doesn't override the allocator
cargo run -p test-dylib --features dylib-jemalloc # Works on current, but doesn't override the allocator